diff --git a/src/BtDependency.cc b/src/BtDependency.cc
index a952feae..8862e9c3 100644
--- a/src/BtDependency.cc
+++ b/src/BtDependency.cc
@@ -155,20 +155,20 @@ bool BtDependency::resolve()
       }
     } catch(RecoverableException& e) {
       A2_LOG_ERROR_EX(EX_EXCEPTION_CAUGHT, e);
-      A2_LOG_INFO(fmt("BtDependency for GID#%" PRId64 " failed. Go without Bt.",
-                      dependant_->getGID()));
+      A2_LOG_INFO(fmt("BtDependency for GID#%s failed. Go without Bt.",
+                      GroupId::toHex(dependant_->getGID()).c_str()));
       return true;
     }
-    A2_LOG_INFO(fmt("Dependency resolved for GID#%" PRId64 "",
-                    dependant_->getGID()));
+    A2_LOG_INFO(fmt("Dependency resolved for GID#%s",
+                    GroupId::toHex(dependant_->getGID()).c_str()));
     dependant_->setDownloadContext(context);
     return true;
   } else if(dependee_->getNumCommand() == 0) {
     // dependee_'s download failed.
     // cut reference here
     dependee_.reset();
-    A2_LOG_INFO(fmt("BtDependency for GID#%" PRId64 " failed. Go without Bt.",
-                    dependant_->getGID()));
+    A2_LOG_INFO(fmt("BtDependency for GID#%s failed. Go without Bt.",
+                    GroupId::toHex(dependant_->getGID()).c_str()));
     return true;
   } else {
     return false;
diff --git a/src/BtStopDownloadCommand.cc b/src/BtStopDownloadCommand.cc
index b6e28cb6..3fe63b5a 100644
--- a/src/BtStopDownloadCommand.cc
+++ b/src/BtStopDownloadCommand.cc
@@ -62,9 +62,9 @@ void BtStopDownloadCommand::preProcess()
     enableExit();
   }
   if(checkPoint_.difference(global::wallclock()) >= timeout_) {
-    A2_LOG_NOTICE(fmt(_("GID#%" PRId64 " Stop downloading torrent due to"
+    A2_LOG_NOTICE(fmt(_("GID#%s Stop downloading torrent due to"
                         " --bt-stop-timeout option."),
-                      requestGroup_->getGID()));
+                      GroupId::toHex(requestGroup_->getGID()).c_str()));
     requestGroup_->setForceHaltRequested(true);
     getDownloadEngine()->setRefreshInterval(0);
     enableExit();
diff --git a/src/ConsoleStatCalc.cc b/src/ConsoleStatCalc.cc
index 861fece8..8893fea6 100644
--- a/src/ConsoleStatCalc.cc
+++ b/src/ConsoleStatCalc.cc
@@ -146,7 +146,7 @@ void printProgressCompact(std::ostream& o, const DownloadEngine* e,
         i = groups.begin(), eoi = groups.end(); i != eoi && cnt < MAX_ITEM;
       ++i, ++cnt) {
     TransferStat stat = (*i)->calculateStat();
-    o << "[#" << (*i)->getGID() << " ";
+    o << "[#" << GroupId::toAbbrevHex((*i)->getGID()) << " ";
     printSizeProgress(o, *i, stat, sizeFormatter);
     o << "]";
   }
@@ -166,7 +166,7 @@ void printProgress
   if(rg->getTotalLength() > 0 && stat.downloadSpeed > 0) {
     eta = (rg->getTotalLength()-rg->getCompletedLength())/stat.downloadSpeed;
   }
-  o << "[#" << rg->getGID() << " ";
+  o << "[#" << GroupId::toAbbrevHex(rg->getGID()) << " ";
   printSizeProgress(o, rg, stat, sizeFormatter);
   o << " CN:"
     << rg->getNumConnection();
@@ -330,7 +330,7 @@ ConsoleStatCalc::calculateStat(const DownloadEngine* e)
       e->getFileAllocationMan()->getPickedEntry();
     if(entry) {
       o << " [FileAlloc:#"
-        << entry->getRequestGroup()->getGID() << " "
+        << GroupId::toAbbrevHex(entry->getRequestGroup()->getGID()) << " "
         << sizeFormatter(entry->getCurrentLength()) << "B/"
         << sizeFormatter(entry->getTotalLength()) << "B(";
       if(entry->getTotalLength() > 0) {
@@ -350,7 +350,7 @@ ConsoleStatCalc::calculateStat(const DownloadEngine* e)
       e->getCheckIntegrityMan()->getPickedEntry();
     if(entry) {
       o << " [Checksum:#"
-        << entry->getRequestGroup()->getGID() << " "
+        << GroupId::toAbbrevHex(entry->getRequestGroup()->getGID()) << " "
         << sizeFormatter(entry->getCurrentLength()) << "B/"
         << sizeFormatter(entry->getTotalLength()) << "B(";
       if(entry->getTotalLength() > 0) {
diff --git a/src/DownloadResult.h b/src/DownloadResult.h
index fa7e3f51..b6972ad0 100644
--- a/src/DownloadResult.h
+++ b/src/DownloadResult.h
@@ -54,7 +54,7 @@ class MetadataInfo;
 
 struct DownloadResult
 {
-  a2_gid_t gid;
+  SharedHandle<GroupId> gid;
 
   std::vector<SharedHandle<FileEntry> > fileEntries;
 
diff --git a/src/FtpNegotiationCommand.cc b/src/FtpNegotiationCommand.cc
index 362f94e7..6da4dc27 100644
--- a/src/FtpNegotiationCommand.cc
+++ b/src/FtpNegotiationCommand.cc
@@ -423,7 +423,7 @@ bool FtpNegotiationCommand::onFileSizeDetermined(int64_t totalLength)
           sequence_ = SEQ_DOWNLOAD_ALREADY_COMPLETED;
           A2_LOG_NOTICE
             (fmt(MSG_DOWNLOAD_ALREADY_COMPLETED,
-                 getRequestGroup()->getGID(),
+                 GroupId::toHex(getRequestGroup()->getGID()).c_str(),
                  getRequestGroup()->getFirstFilePath().c_str()));
         }
       poolConnection();
diff --git a/src/GroupId.cc b/src/GroupId.cc
new file mode 100644
index 00000000..a5c9d4ea
--- /dev/null
+++ b/src/GroupId.cc
@@ -0,0 +1,158 @@
+/* <!-- copyright */
+/*
+ * aria2 - The high speed download utility
+ *
+ * Copyright (C) 2012 Tatsuhiro Tsujikawa
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * In addition, as a special exception, the copyright holders give
+ * permission to link the code of portions of this program with the
+ * OpenSSL library under certain conditions as described in each
+ * individual source file, and distribute linked combinations
+ * including the two.
+ * You must obey the GNU General Public License in all respects
+ * for all of the code used other than OpenSSL.  If you modify
+ * file(s) with this exception, you may extend this exception to your
+ * version of the file(s), but you are not obligated to do so.  If you
+ * do not wish to do so, delete this exception statement from your
+ * version.  If you delete this exception statement from all source
+ * files in the program, then also delete it here.
+ */
+/* copyright --> */
+#include "GroupId.h"
+#include "util.h"
+
+namespace aria2 {
+
+std::set<a2_gid_t> GroupId::set_;
+
+SharedHandle<GroupId> GroupId::create()
+{
+  a2_gid_t n;
+  for(;;) {
+    util::generateRandomData(reinterpret_cast<unsigned char*>(&n), sizeof(n));
+    if(n != 0 && set_.count(n) == 0) {
+      break;
+    }
+  }
+  SharedHandle<GroupId> res(new GroupId(n));
+  return res;
+}
+
+SharedHandle<GroupId> GroupId::import(a2_gid_t n)
+{
+  SharedHandle<GroupId> res;
+  if(n == 0 || set_.count(n) != 0) {
+    return res;
+  }
+  res.reset(new GroupId(n));
+  return res;
+}
+
+void GroupId::clear()
+{
+  set_.clear();
+}
+
+int GroupId::expandUnique(a2_gid_t& n, const char* hex)
+{
+  a2_gid_t p = 0;
+  size_t i;
+  for(i = 0; hex[i]; ++i) {
+    unsigned int c = util::hexCharToUInt(hex[i]);
+    if(c == 255) {
+      return ERR_INVALID;
+    }
+    p <<= 4;
+    p |= c;
+  }
+  if(i == 0 || i > sizeof(a2_gid_t)*2) {
+    return ERR_INVALID;
+  }
+  p <<= 64-i*4;
+  a2_gid_t mask = UINT64_MAX-((1LL << (64-i*4))-1);
+  std::set<a2_gid_t>::const_iterator itr = set_.lower_bound(p);
+  if(itr == set_.end()) {
+    return ERR_NOT_FOUND;
+  }
+  if(p == ((*itr)&mask)) {
+    n = *itr;
+    ++itr;
+    if(itr == set_.end() || p != ((*itr)&mask)) {
+      return 0;
+    } else {
+      return ERR_NOT_UNIQUE;
+    }
+  } else {
+    return ERR_NOT_FOUND;
+  }
+}
+
+int GroupId::toNumericId(a2_gid_t& n, const char* hex)
+{
+  a2_gid_t p = 0;
+  size_t i;
+  for(i = 0; hex[i]; ++i) {
+    unsigned int c = util::hexCharToUInt(hex[i]);
+    if(c == 255) {
+      return ERR_INVALID;
+    }
+    p <<= 4;
+    p |= c;
+  }
+  if(p == 0 || i != sizeof(a2_gid_t)*2) {
+    return ERR_INVALID;
+  }
+  n = p;
+  return 0;
+}
+
+std::string GroupId::toHex(a2_gid_t gid)
+{
+  a2_gid_t n = hton64(gid);
+  return util::toHex(reinterpret_cast<unsigned char*>(&n), sizeof(n));
+}
+
+std::string GroupId::toAbbrevHex(a2_gid_t gid)
+{
+  const size_t abbrevSize = 6;
+  std::string h = toHex(gid);
+  assert(h.size() >= abbrevSize);
+  return toHex(gid).erase(abbrevSize);
+}
+
+std::string GroupId::toHex() const
+{
+  return toHex(gid_);
+}
+
+std::string GroupId::toAbbrevHex() const
+{
+  return toAbbrevHex(gid_);
+}
+
+GroupId::GroupId(a2_gid_t gid)
+  : gid_(gid)
+{
+  set_.insert(gid_);
+}
+
+GroupId::~GroupId()
+{
+  set_.erase(gid_);
+}
+
+} // namespace aria2
diff --git a/src/GroupId.h b/src/GroupId.h
new file mode 100644
index 00000000..25e3c4f2
--- /dev/null
+++ b/src/GroupId.h
@@ -0,0 +1,81 @@
+/* <!-- copyright */
+/*
+ * aria2 - The high speed download utility
+ *
+ * Copyright (C) 2012 Tatsuhiro Tsujikawa
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * In addition, as a special exception, the copyright holders give
+ * permission to link the code of portions of this program with the
+ * OpenSSL library under certain conditions as described in each
+ * individual source file, and distribute linked combinations
+ * including the two.
+ * You must obey the GNU General Public License in all respects
+ * for all of the code used other than OpenSSL.  If you modify
+ * file(s) with this exception, you may extend this exception to your
+ * version of the file(s), but you are not obligated to do so.  If you
+ * do not wish to do so, delete this exception statement from your
+ * version.  If you delete this exception statement from all source
+ * files in the program, then also delete it here.
+ */
+/* copyright --> */
+#ifndef D_GROUP_ID_H
+#define D_GROUP_ID_H
+
+#include "common.h"
+
+#include <set>
+#include <string>
+
+#include "SharedHandle.h"
+
+namespace aria2 {
+
+typedef uint64_t a2_gid_t;
+
+class GroupId {
+public:
+  static SharedHandle<GroupId> create();
+  static SharedHandle<GroupId> import(a2_gid_t n);
+  static void clear();
+  enum {
+    ERR_NOT_UNIQUE = -1,
+    ERR_NOT_FOUND = -2,
+    ERR_INVALID = -3
+  };
+  static int expandUnique(a2_gid_t& n, const char* hex);
+  static int toNumericId(a2_gid_t& n, const char* hex);
+  static std::string toHex(a2_gid_t n);
+  static std::string toAbbrevHex(a2_gid_t n);
+
+  ~GroupId();
+  a2_gid_t getNumericId() const
+  {
+    return gid_;
+  }
+  std::string toHex() const;
+  std::string toAbbrevHex() const;
+private:
+  static std::set<a2_gid_t> set_;
+
+  GroupId(a2_gid_t gid);
+
+  a2_gid_t gid_;
+};
+
+} // namespace aria2
+
+#endif // D_GROUP_ID_H
diff --git a/src/HttpResponseCommand.cc b/src/HttpResponseCommand.cc
index e85a0329..c04e5818 100644
--- a/src/HttpResponseCommand.cc
+++ b/src/HttpResponseCommand.cc
@@ -189,7 +189,7 @@ bool HttpResponseCommand::executeInternal()
     getDownloadContext()->setChecksumVerified(true);
     A2_LOG_NOTICE
       (fmt(MSG_DOWNLOAD_ALREADY_COMPLETED,
-           getRequestGroup()->getGID(),
+           GroupId::toHex(getRequestGroup()->getGID()).c_str(),
            getRequestGroup()->getFirstFilePath().c_str()));
     poolConnection();
     getFileEntry()->poolRequest(getRequest());
@@ -453,7 +453,7 @@ bool HttpResponseCommand::handleOtherEncoding
         getDownloadContext()->setChecksumVerified(true);
         A2_LOG_NOTICE
           (fmt(MSG_DOWNLOAD_ALREADY_COMPLETED,
-               getRequestGroup()->getGID(),
+               GroupId::toHex(getRequestGroup()->getGID()).c_str(),
                getRequestGroup()->getFirstFilePath().c_str()));
       }
     poolConnection();
diff --git a/src/Makefile.am b/src/Makefile.am
index 2e9217ef..656af6f5 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -242,7 +242,8 @@ SRCS =  SocketCore.cc SocketCore.h\
 	AnonDiskWriterFactory.h\
 	XmlRpcRequestParserController.cc XmlRpcRequestParserController.h\
 	WrDiskCache.cc WrDiskCache.h\
-	WrDiskCacheEntry.cc WrDiskCacheEntry.h
+	WrDiskCacheEntry.cc WrDiskCacheEntry.h\
+	GroupId.cc GroupId.h
 
 if MINGW_BUILD
 SRCS += WinConsoleFile.cc WinConsoleFile.h
diff --git a/src/Metalink2RequestGroup.cc b/src/Metalink2RequestGroup.cc
index 6f15f85e..3a282618 100644
--- a/src/Metalink2RequestGroup.cc
+++ b/src/Metalink2RequestGroup.cc
@@ -241,7 +241,7 @@ Metalink2RequestGroup::createRequestGroup
     }
 #endif // ENABLE_BITTORRENT
     SharedHandle<Option> option = util::copy(optionTemplate);
-    SharedHandle<RequestGroup> rg(new RequestGroup(option));
+    SharedHandle<RequestGroup> rg(new RequestGroup(GroupId::create(), option));
     SharedHandle<DownloadContext> dctx;
     int numSplit = option->getAsInt(PREF_SPLIT);
     int maxConn = option->getAsInt(PREF_MAX_CONNECTION_PER_SERVER);
diff --git a/src/RequestGroup.cc b/src/RequestGroup.cc
index c296c321..1776e548 100644
--- a/src/RequestGroup.cc
+++ b/src/RequestGroup.cc
@@ -120,10 +120,9 @@
 
 namespace aria2 {
 
-a2_gid_t RequestGroup::gidCounter_ = 0;
-
-RequestGroup::RequestGroup(const SharedHandle<Option>& option)
-  : gid_(newGID()),
+RequestGroup::RequestGroup(const SharedHandle<GroupId>& gid,
+                           const SharedHandle<Option>& option)
+  : gid_(gid),
     state_(STATE_WAITING),
     option_(option),
     numConcurrentCommand_(option->getAsInt(PREF_SPLIT)),
@@ -244,7 +243,8 @@ SharedHandle<CheckIntegrityEntry> RequestGroup::createCheckIntegrityEntry()
         {
           downloadContext_->setChecksumVerified(true);
           A2_LOG_NOTICE(fmt(MSG_DOWNLOAD_ALREADY_COMPLETED,
-                            gid_, downloadContext_->getBasePath().c_str()));
+                            gid_->toHex().c_str(),
+                            downloadContext_->getBasePath().c_str()));
         }
     } else {
       checkEntry.reset(new StreamCheckIntegrityEntry(this));
@@ -346,9 +346,9 @@ void RequestGroup::createInitialCommand
       btAnnouncePtr->shuffleAnnounce();
       SharedHandle<BtAnnounce> btAnnounce(btAnnouncePtr);
 
-      assert(!btRegistry->get(gid_));
+      assert(!btRegistry->get(gid_->getNumericId()));
       btRegistry->put
-        (gid_, SharedHandle<BtObject>
+        (gid_->getNumericId(), SharedHandle<BtObject>
          (new BtObject
           (downloadContext_,
            pieceStorage_,
@@ -962,7 +962,7 @@ void RequestGroup::decreaseNumCommand()
 {
   --numCommand_;
   if(!numCommand_ && requestGroupMan_) {
-    A2_LOG_DEBUG(fmt("GID#%" PRId64 " - Request queue check", gid_));
+    A2_LOG_DEBUG(fmt("GID#%s - Request queue check", gid_->toHex().c_str()));
     requestGroupMan_->requestQueueCheck();
   }
 }
@@ -1007,7 +1007,7 @@ void RequestGroup::setPauseRequested(bool f)
 void RequestGroup::releaseRuntimeResource(DownloadEngine* e)
 {
 #ifdef ENABLE_BITTORRENT
-  e->getBtRegistry()->remove(gid_);
+  e->getBtRegistry()->remove(gid_->getNumericId());
   btRuntime_ = 0;
   peerStorage_ = 0;
 #endif // ENABLE_BITTORRENT
@@ -1152,7 +1152,8 @@ bool RequestGroup::needsFileAllocation() const
 
 SharedHandle<DownloadResult> RequestGroup::createDownloadResult() const
 {
-  A2_LOG_DEBUG(fmt("GID#%" PRId64 " - Creating DownloadResult.", gid_));
+  A2_LOG_DEBUG(fmt("GID#%s - Creating DownloadResult.",
+                   gid_->toHex().c_str()));
   TransferStat st = calculateStat();
   SharedHandle<DownloadResult> res(new DownloadResult());
   res->gid = gid_;
@@ -1298,12 +1299,4 @@ bool RequestGroup::p2pInvolved() const
 #endif // !ENABLE_BITTORRENT
 }
 
-a2_gid_t RequestGroup::newGID()
-{
-  if(gidCounter_ == INT64_MAX) {
-    gidCounter_ = 0;
-  }
-  return ++gidCounter_;
-}
-
 } // namespace aria2
diff --git a/src/RequestGroup.h b/src/RequestGroup.h
index 321d4d90..df35c19e 100644
--- a/src/RequestGroup.h
+++ b/src/RequestGroup.h
@@ -47,6 +47,7 @@
 #include "Request.h"
 #include "error_code.h"
 #include "MetadataInfo.h"
+#include "GroupId.h"
 
 namespace aria2 {
 
@@ -73,8 +74,6 @@ class BtRuntime;
 class PeerStorage;
 #endif // ENABLE_BITTORRENT
 
-typedef int64_t a2_gid_t;
-
 class RequestGroup {
 public:
   enum HaltReason {
@@ -89,9 +88,7 @@ public:
     STATE_ACTIVE
   };
 private:
-  static a2_gid_t gidCounter_;
-
-  a2_gid_t gid_;
+  SharedHandle<GroupId> gid_;
 
   int state_;
 
@@ -201,7 +198,8 @@ private:
   (const SharedHandle<BtProgressInfoFile>& progressInfoFile);
 
 public:
-  RequestGroup(const SharedHandle<Option>& option);
+  RequestGroup(const SharedHandle<GroupId>& gid,
+               const SharedHandle<Option>& option);
 
   ~RequestGroup();
 
@@ -264,7 +262,7 @@ public:
 
   a2_gid_t getGID() const
   {
-    return gid_;
+    return gid_->getNumericId();
   }
 
   TransferStat calculateStat() const;
@@ -550,10 +548,6 @@ public:
   {
     state_ = state;
   }
-
-  static void resetGIDCounter() { gidCounter_ = 0; }
-
-  static a2_gid_t newGID();
 };
 
 } // namespace aria2
diff --git a/src/RequestGroupMan.cc b/src/RequestGroupMan.cc
index 86a26b39..30b7111c 100644
--- a/src/RequestGroupMan.cc
+++ b/src/RequestGroupMan.cc
@@ -261,7 +261,8 @@ size_t RequestGroupMan::changeReservedGroupPosition
   std::deque<SharedHandle<RequestGroup> >::iterator i =
     findByGID(reservedGroups_.begin(), reservedGroups_.end(), gid);
   if(i == reservedGroups_.end()) {
-    throw DL_ABORT_EX(fmt("GID#%" PRId64 " not found in the waiting queue.", gid));
+    throw DL_ABORT_EX(fmt("GID#%s not found in the waiting queue.",
+                          GroupId::toHex(gid).c_str()));
   }
   SharedHandle<RequestGroup> rg = *i;
   const size_t maxPos = reservedGroups_.size()-1;
@@ -401,7 +402,8 @@ public:
         group->closeFile();
         if(group->isPauseRequested()) {
           A2_LOG_NOTICE
-            (fmt(_("Download GID#%" PRId64 " paused"), group->getGID()));
+            (fmt(_("Download GID#%s paused"),
+                 GroupId::toHex(group->getGID()).c_str()));
           group->saveControlFile();
         } else if(group->downloadFinished() &&
            !group->getDownloadContext()->isChecksumVerificationNeeded()) {
@@ -429,7 +431,8 @@ public:
           if(group->getOption()->getAsBool(PREF_BT_REMOVE_UNSELECTED_FILE) &&
              !group->inMemoryDownload() &&
              dctx->hasAttribute(CTX_ATTR_BT)) {
-            A2_LOG_INFO(fmt(MSG_REMOVING_UNSELECTED_FILE, group->getGID()));
+            A2_LOG_INFO(fmt(MSG_REMOVING_UNSELECTED_FILE,
+                            GroupId::toHex(group->getGID()).c_str()));
             const std::vector<SharedHandle<FileEntry> >& files =
               dctx->getFileEntries();
             for(std::vector<SharedHandle<FileEntry> >::const_iterator i =
@@ -447,8 +450,8 @@ public:
 #endif // ENABLE_BITTORRENT
         } else {
           A2_LOG_NOTICE
-            (fmt(_("Download GID#%" PRId64 " not complete: %s"),
-                 group->getGID(),
+            (fmt(_("Download GID#%s not complete: %s"),
+                 GroupId::toHex(group->getGID()).c_str(),
                  group->getDownloadContext()->getBasePath().c_str()));
           group->saveControlFile();
         }
@@ -760,24 +763,20 @@ const char* getStatusStr(DownloadStatus status, bool useColor)
 
 void RequestGroupMan::showDownloadResults(OutputFile& o, bool full) const
 {
-#ifdef __MINGW32__
-  int pathRowSize = 58;
-#else // !__MINGW32__
-  int pathRowSize = 59;
-#endif // !__MINGW32__
+  int pathRowSize = 55;
   // Download Results:
   // idx|stat|path/length
   // ===+====+=======================================================================
   o.printf("\n%s"
-           "\ngid|stat|avg speed  |",
+           "\ngid   |stat|avg speed  |",
            _("Download Results:"));
   if(full) {
     o.write("  %|path/URI"
-            "\n===+====+===========+===+");
+            "\n======+====+===========+===+");
     pathRowSize -= 4;
   } else {
     o.write("path/URI"
-            "\n===+====+===========+");
+            "\n======+====+===========+");
   }
   std::string line(pathRowSize, '=');
   o.printf("%s\n", line.c_str());
@@ -837,7 +836,7 @@ void formatDownloadResultCommon
  const char* status,
  const SharedHandle<DownloadResult>& downloadResult)
 {
-  o << std::setw(3) << downloadResult->gid << "|"
+  o << std::setw(3) << downloadResult->gid->toAbbrevHex() << "|"
     << std::setw(4) << status << "|"
     << std::setw(11);
   if(downloadResult->sessionTime > 0) {
@@ -975,7 +974,7 @@ RequestGroupMan::findDownloadResult(a2_gid_t gid) const
 {
   for(std::deque<SharedHandle<DownloadResult> >::const_iterator i =
         downloadResults_.begin(), eoi = downloadResults_.end(); i != eoi; ++i) {
-    if((*i)->gid == gid) {
+    if((*i)->gid->getNumericId() == gid) {
       return *i;
     }
   }
@@ -986,7 +985,7 @@ bool RequestGroupMan::removeDownloadResult(a2_gid_t gid)
 {
   for(std::deque<SharedHandle<DownloadResult> >::iterator i =
         downloadResults_.begin(), eoi = downloadResults_.end(); i != eoi; ++i) {
-    if((*i)->gid == gid) {
+    if((*i)->gid->getNumericId() == gid) {
       downloadResults_.erase(i);
       return true;
     }
diff --git a/src/RpcMethodImpl.cc b/src/RpcMethodImpl.cc
index 88940ba4..d93e3c8e 100644
--- a/src/RpcMethodImpl.cc
+++ b/src/RpcMethodImpl.cc
@@ -148,7 +148,7 @@ const std::string KEY_NUM_ACTIVE = "numActive";
 namespace {
 SharedHandle<ValueBase> createGIDResponse(a2_gid_t gid)
 {
-  return String::g(util::itos(gid));
+  return String::g(GroupId::toHex(gid));
 }
 } // namespace
 
@@ -185,12 +185,19 @@ namespace {
 a2_gid_t str2Gid(const String* str)
 {
   assert(str);
-  int64_t gid;
-  if(util::parseLLIntNoThrow(gid, str->s())) {
-    return gid;
-  } else {
-    throw DL_ABORT_EX(fmt("Bad GID %s", str->s().c_str()));
+  if(str->s().size() > sizeof(a2_gid_t)*2) {
+    throw DL_ABORT_EX(fmt("Invalid GID %s", str->s().c_str()));
   }
+  a2_gid_t n;
+  switch(GroupId::expandUnique(n, str->s().c_str())) {
+  case GroupId::ERR_NOT_UNIQUE:
+    throw DL_ABORT_EX(fmt("GID %s is not unique", str->s().c_str()));
+  case GroupId::ERR_NOT_FOUND:
+    throw DL_ABORT_EX(fmt("GID %s is not found", str->s().c_str()));
+  case GroupId::ERR_INVALID:
+    throw DL_ABORT_EX(fmt("Invalid GID %s", str->s().c_str()));
+  }
+  return n;
 }
 } // namespace
 
@@ -364,7 +371,7 @@ SharedHandle<ValueBase> AddMetalinkRpcMethod::process
     }
     for(std::vector<SharedHandle<RequestGroup> >::const_iterator i =
           result.begin(), eoi = result.end(); i != eoi; ++i) {
-      gids->append(util::itos((*i)->getGID()));
+      gids->append(GroupId::toHex((*i)->getGID()));
     }
   }
   return gids;
@@ -391,12 +398,13 @@ SharedHandle<ValueBase> removeDownload
       if(group->isDependencyResolved()) {
         e->getRequestGroupMan()->removeReservedGroup(gid);
       } else {
-        throw DL_ABORT_EX(fmt("GID#%" PRId64 " cannot be removed now", gid));
+        throw DL_ABORT_EX(fmt("GID#%s cannot be removed now",
+                              GroupId::toHex(gid).c_str()));
       }
     }
   } else {
-    throw DL_ABORT_EX(fmt("Active Download not found for GID#%" PRId64,
-                          gid));
+    throw DL_ABORT_EX(fmt("Active Download not found for GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   return createGIDResponse(gid);
 }
@@ -455,7 +463,8 @@ SharedHandle<ValueBase> pauseDownload
       return createGIDResponse(gid);
     }
   }
-  throw DL_ABORT_EX(fmt("GID#%" PRId64 " cannot be paused now", gid));
+  throw DL_ABORT_EX(fmt("GID#%s cannot be paused now",
+                        GroupId::toHex(gid).c_str()));
 }
 } // namespace
 
@@ -519,7 +528,8 @@ SharedHandle<ValueBase> UnpauseRpcMethod::process
   if(!group ||
      group->getState() != RequestGroup::STATE_WAITING ||
      !group->isPauseRequested()) {
-    throw DL_ABORT_EX(fmt("GID#%" PRId64 " cannot be unpaused now", gid));
+    throw DL_ABORT_EX(fmt("GID#%s cannot be unpaused now",
+                          GroupId::toHex(gid).c_str()));
   } else {
     group->setPauseRequested(false);
     e->getRequestGroupMan()->requestQueueCheck();
@@ -644,7 +654,7 @@ void gatherProgressCommon
 {
   const SharedHandle<PieceStorage>& ps = group->getPieceStorage();
   if(requested_key(keys, KEY_GID)) {
-    entryDict->put(KEY_GID, util::itos(group->getGID()));
+    entryDict->put(KEY_GID, GroupId::toHex(group->getGID()).c_str());
   }
   if(requested_key(keys, KEY_TOTAL_LENGTH)) {
     // This is "filtered" total length if --select-file is used.
@@ -690,14 +700,14 @@ void gatherProgressCommon
       // The element is GID.
       for(std::vector<a2_gid_t>::const_iterator i = group->followedBy().begin(),
             eoi = group->followedBy().end(); i != eoi; ++i) {
-        list->append(util::itos(*i));
+        list->append(GroupId::toHex(*i));
       }
       entryDict->put(KEY_FOLLOWED_BY, list);
     }
   }
   if(requested_key(keys, KEY_BELONGS_TO)) {
     if(group->belongsTo()) {
-      entryDict->put(KEY_BELONGS_TO, util::itos(group->belongsTo()));
+      entryDict->put(KEY_BELONGS_TO, GroupId::toHex(group->belongsTo()));
     }
   }
   if(requested_key(keys, KEY_FILES)) {
@@ -832,7 +842,7 @@ void gatherStoppedDownload
  const std::vector<std::string>& keys)
 {
   if(requested_key(keys, KEY_GID)) {
-    entryDict->put(KEY_GID, util::itos(ds->gid));
+    entryDict->put(KEY_GID, ds->gid->toHex());
   }
   if(requested_key(keys, KEY_ERROR_CODE)) {
     entryDict->put(KEY_ERROR_CODE, util::itos(static_cast<int>(ds->result)));
@@ -852,14 +862,14 @@ void gatherStoppedDownload
       // The element is GID.
       for(std::vector<a2_gid_t>::const_iterator i = ds->followedBy.begin(),
             eoi = ds->followedBy.end(); i != eoi; ++i) {
-        list->append(util::itos(*i));
+        list->append(GroupId::toHex(*i));
       }
       entryDict->put(KEY_FOLLOWED_BY, list);
     }
   }
   if(requested_key(keys, KEY_BELONGS_TO)) {
     if(ds->belongsTo) {
-      entryDict->put(KEY_BELONGS_TO, util::itos(ds->belongsTo));
+      entryDict->put(KEY_BELONGS_TO, GroupId::toHex(ds->belongsTo));
     }
   }
   if(requested_key(keys, KEY_FILES)) {
@@ -922,8 +932,8 @@ SharedHandle<ValueBase> GetFilesRpcMethod::process
     SharedHandle<DownloadResult> dr =
       e->getRequestGroupMan()->findDownloadResult(gid);
     if(!dr) {
-      throw DL_ABORT_EX(fmt("No file data is available for GID#%" PRId64 "",
-                            gid));
+      throw DL_ABORT_EX(fmt("No file data is available for GID#%s",
+                            GroupId::toHex(gid).c_str()));
     } else {
       createFileEntry(files, dr->fileEntries.begin(), dr->fileEntries.end(),
                       dr->totalLength, dr->pieceLength, dr->bitfield);
@@ -949,7 +959,8 @@ SharedHandle<ValueBase> GetUrisRpcMethod::process
   a2_gid_t gid = str2Gid(gidParam);
   SharedHandle<RequestGroup> group = e->getRequestGroupMan()->findGroup(gid);
   if(!group) {
-    throw DL_ABORT_EX(fmt("No URI data is available for GID#%" PRId64, gid));
+    throw DL_ABORT_EX(fmt("No URI data is available for GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   SharedHandle<List> uriList = List::g();
   // TODO Current implementation just returns first FileEntry's URIs.
@@ -968,7 +979,8 @@ SharedHandle<ValueBase> GetPeersRpcMethod::process
   a2_gid_t gid = str2Gid(gidParam);
   SharedHandle<RequestGroup> group = e->getRequestGroupMan()->findGroup(gid);
   if(!group) {
-    throw DL_ABORT_EX(fmt("No peer data is available for GID#%" PRId64, gid));
+    throw DL_ABORT_EX(fmt("No peer data is available for GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   SharedHandle<List> peers = List::g();
   const SharedHandle<BtObject>& btObject =
@@ -998,7 +1010,8 @@ SharedHandle<ValueBase> TellStatusRpcMethod::process
     SharedHandle<DownloadResult> ds =
       e->getRequestGroupMan()->findDownloadResult(gid);
     if(!ds) {
-      throw DL_ABORT_EX(fmt("No such download for GID#%" PRId64 "", gid));
+      throw DL_ABORT_EX(fmt("No such download for GID#%s",
+                            GroupId::toHex(gid).c_str()));
     }
     gatherStoppedDownload(entryDict, ds, keys);
   } else {
@@ -1090,7 +1103,8 @@ SharedHandle<ValueBase> RemoveDownloadResultRpcMethod::process
 
   a2_gid_t gid = str2Gid(gidParam);
   if(!e->getRequestGroupMan()->removeDownloadResult(gid)) {
-    throw DL_ABORT_EX(fmt("Could not remove download result of GID#%" PRId64 "", gid));
+    throw DL_ABORT_EX(fmt("Could not remove download result of GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   return VLB_OK;
 }
@@ -1191,7 +1205,8 @@ SharedHandle<ValueBase> ChangeOptionRpcMethod::process
     }
     changeOption(group, option, e);
   } else {
-    throw DL_ABORT_EX(fmt("Cannot change option for GID#%" PRId64, gid));
+    throw DL_ABORT_EX(fmt("Cannot change option for GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   return VLB_OK;
 }
@@ -1277,7 +1292,8 @@ SharedHandle<ValueBase> GetOptionRpcMethod::process
   a2_gid_t gid = str2Gid(gidParam);
   SharedHandle<RequestGroup> group = e->getRequestGroupMan()->findGroup(gid);
   if(!group) {
-    throw DL_ABORT_EX(fmt("Cannot get option for GID#%" PRId64 "", gid));
+    throw DL_ABORT_EX(fmt("Cannot get option for GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   SharedHandle<Dict> result = Dict::g();
   SharedHandle<Option> option = group->getOption();
@@ -1344,7 +1360,8 @@ SharedHandle<ValueBase> GetServersRpcMethod::process
   a2_gid_t gid = str2Gid(gidParam);
   SharedHandle<RequestGroup> group = e->getRequestGroupMan()->findGroup(gid);
   if(!group || group->getState() != RequestGroup::STATE_ACTIVE) {
-    throw DL_ABORT_EX(fmt("No active download for GID#%" PRId64, gid));
+    throw DL_ABORT_EX(fmt("No active download for GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   const SharedHandle<DownloadContext>& dctx = group->getDownloadContext();
   const std::vector<SharedHandle<FileEntry> >& files = dctx->getFileEntries();
@@ -1390,7 +1407,8 @@ SharedHandle<ValueBase> ChangeUriRpcMethod::process
   size_t index = indexParam->i()-1;
   SharedHandle<RequestGroup> group = e->getRequestGroupMan()->findGroup(gid);
   if(!group) {
-    throw DL_ABORT_EX(fmt("Cannot remove URIs from GID#%" PRId64 "", gid));
+    throw DL_ABORT_EX(fmt("Cannot remove URIs from GID#%s",
+                          GroupId::toHex(gid).c_str()));
   }
   const SharedHandle<DownloadContext>& dctx = group->getDownloadContext();
   const std::vector<SharedHandle<FileEntry> >& files = dctx->getFileEntries();
diff --git a/src/TrackerWatcherCommand.cc b/src/TrackerWatcherCommand.cc
index 00822a4f..fcd3b6f2 100644
--- a/src/TrackerWatcherCommand.cc
+++ b/src/TrackerWatcherCommand.cc
@@ -221,7 +221,7 @@ TrackerWatcherCommand::createRequestGroup(const std::string& uri)
   std::vector<std::string> uris;
   uris.push_back(uri);
   SharedHandle<Option> option = util::copy(getOption());
-  SharedHandle<RequestGroup> rg(new RequestGroup(option));
+  SharedHandle<RequestGroup> rg(new RequestGroup(GroupId::create(), option));
   if(backupTrackerIsAvailable(requestGroup_->getDownloadContext())) {
     A2_LOG_DEBUG("This is multi-tracker announce.");
   } else {
@@ -255,7 +255,8 @@ TrackerWatcherCommand::createRequestGroup(const std::string& uri)
   rg->clearPreDownloadHandler();
   rg->clearPostDownloadHandler();
   dctx->setAcceptMetalink(false);
-  A2_LOG_INFO(fmt("Creating tracker request group GID#%" PRId64 "", rg->getGID()));
+  A2_LOG_INFO(fmt("Creating tracker request group GID#%s",
+                  GroupId::toHex(rg->getGID()).c_str()));
   return rg;
 }
 
diff --git a/src/WebSocketSessionMan.cc b/src/WebSocketSessionMan.cc
index 15f6c935..26771bc4 100644
--- a/src/WebSocketSessionMan.cc
+++ b/src/WebSocketSessionMan.cc
@@ -66,7 +66,7 @@ void WebSocketSessionMan::addNotification
   dict->put("jsonrpc", "2.0");
   dict->put("method", method);
   SharedHandle<Dict> eventSpec = Dict::g();
-  eventSpec->put("gid", util::itos(group->getGID()));
+  eventSpec->put("gid", GroupId::toHex((group->getGID())));
   SharedHandle<List> params = List::g();
   params->append(eventSpec);
   dict->put("params", params);
diff --git a/src/download_helper.cc b/src/download_helper.cc
index 092ba544..5a448dcf 100644
--- a/src/download_helper.cc
+++ b/src/download_helper.cc
@@ -112,7 +112,7 @@ SharedHandle<RequestGroup> createRequestGroup
  bool useOutOption = false)
 {
   SharedHandle<Option> option = util::copy(optionTemplate);
-  SharedHandle<RequestGroup> rg(new RequestGroup(option));
+  SharedHandle<RequestGroup> rg(new RequestGroup(GroupId::create(), option));
   SharedHandle<DownloadContext> dctx
     (new DownloadContext
      (option->getAsInt(PREF_PIECE_LENGTH),
@@ -168,7 +168,7 @@ createBtRequestGroup(const std::string& metaInfoUri,
                      bool adjustAnnounceUri = true)
 {
   SharedHandle<Option> option = util::copy(optionTemplate);
-  SharedHandle<RequestGroup> rg(new RequestGroup(option));
+  SharedHandle<RequestGroup> rg(new RequestGroup(GroupId::create(), option));
   SharedHandle<DownloadContext> dctx(new DownloadContext());
   // may throw exception
   bittorrent::loadFromMemory(torrent, dctx, option, auxUris,
@@ -210,7 +210,7 @@ createBtMagnetRequestGroup
  const SharedHandle<Option>& optionTemplate)
 {
   SharedHandle<Option> option = util::copy(optionTemplate);
-  SharedHandle<RequestGroup> rg(new RequestGroup(option));
+  SharedHandle<RequestGroup> rg(new RequestGroup(GroupId::create(), option));
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(METADATA_PIECE_SIZE, 0,
                          A2STR::NIL));
diff --git a/src/message.h b/src/message.h
index 66681dfa..fd6ee70a 100644
--- a/src/message.h
+++ b/src/message.h
@@ -54,7 +54,7 @@
 #define MSG_GOT_NEW_PIECE "CUID#%" PRId64 " - we got new piece. index=%lu"
 #define MSG_GOT_WRONG_PIECE "CUID#%" PRId64 " - we got wrong piece. index=%lu"
 #define MSG_DOWNLOAD_NOT_COMPLETE "CUID#%" PRId64 " - Download not complete: %s"
-#define MSG_DOWNLOAD_ALREADY_COMPLETED _("GID#%" PRId64 " - Download has already completed: %s")
+#define MSG_DOWNLOAD_ALREADY_COMPLETED _("GID#%s - Download has already completed: %s")
 #define MSG_RESOLVING_HOSTNAME "CUID#%" PRId64 " - Resolving hostname %s"
 #define MSG_NAME_RESOLUTION_COMPLETE                    \
   "CUID#%" PRId64 " - Name resolution complete: %s -> %s"
@@ -197,7 +197,7 @@
 #define MSG_DIR_TRAVERSAL_DETECTED _("Detected directory traversal directive in %s")
 #define MSG_HASH_CHECK_NOT_DONE                                         \
   "File has already been downloaded but hash check has not been done yet."
-#define MSG_REMOVING_UNSELECTED_FILE _("GID#%" PRId64 " - Removing unselected file.")
+#define MSG_REMOVING_UNSELECTED_FILE _("GID#%s - Removing unselected file.")
 #define MSG_FILE_REMOVED _("File %s removed.")
 #define MSG_FILE_COULD_NOT_REMOVED _("File %s could not be removed.")
 
diff --git a/src/util.cc b/src/util.cc
index 0f9083b2..da652a8e 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -1691,7 +1691,7 @@ void executeHook
  size_t numFiles,
  const std::string& firstFilename)
 {
-  const std::string gidStr = util::itos(gid);
+  const std::string gidStr = GroupId::toHex(gid);
   const std::string numFilesStr = util::uitos(numFiles);
 #ifndef __MINGW32__
   A2_LOG_INFO(fmt("Executing user command: %s %s %s %s",
diff --git a/test/BtDependencyTest.cc b/test/BtDependencyTest.cc
index c1a18db6..40823b0a 100644
--- a/test/BtDependencyTest.cc
+++ b/test/BtDependencyTest.cc
@@ -38,7 +38,8 @@ class BtDependencyTest:public CppUnit::TestFixture {
 
   SharedHandle<RequestGroup> createDependant(const SharedHandle<Option>& option)
   {
-    SharedHandle<RequestGroup> dependant(new RequestGroup(util::copy(option)));
+    SharedHandle<RequestGroup> dependant(new RequestGroup(GroupId::create(),
+                                                          util::copy(option)));
     SharedHandle<DownloadContext> dctx
       (new DownloadContext(0, 0, "/tmp/outfile.path"));
     std::vector<std::string> uris;
@@ -56,7 +57,8 @@ class BtDependencyTest:public CppUnit::TestFixture {
    const std::string& torrentFile,
    int64_t length)
   {
-    SharedHandle<RequestGroup> dependee(new RequestGroup(util::copy(option)));
+    SharedHandle<RequestGroup> dependee(new RequestGroup(GroupId::create(),
+                                                         util::copy(option)));
     SharedHandle<DownloadContext> dctx
       (new DownloadContext(1024*1024, length, torrentFile));
     dependee->setDownloadContext(dctx);
diff --git a/test/BtPostDownloadHandlerTest.cc b/test/BtPostDownloadHandlerTest.cc
index 606a5c07..3b5e39e6 100644
--- a/test/BtPostDownloadHandlerTest.cc
+++ b/test/BtPostDownloadHandlerTest.cc
@@ -39,7 +39,7 @@ void BtPostDownloadHandlerTest::testCanHandle_extension()
 {
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(0, 0, A2_TEST_DIR"/test.torrent"));
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   BtPostDownloadHandler handler;
@@ -54,7 +54,7 @@ void BtPostDownloadHandlerTest::testCanHandle_contentType()
 {
   SharedHandle<DownloadContext> dctx(new DownloadContext(0, 0, "test"));
   dctx->getFirstFileEntry()->setContentType("application/x-bittorrent");
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   BtPostDownloadHandler handler;
@@ -69,7 +69,7 @@ void BtPostDownloadHandlerTest::testGetNextRequestGroups()
 {
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(1024, 0, A2_TEST_DIR"/test.torrent"));
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
   rg.initPieceStorage();
   rg.getPieceStorage()->getDiskAdaptor()->enableReadOnly();
diff --git a/test/DefaultBtMessageDispatcherTest.cc b/test/DefaultBtMessageDispatcherTest.cc
index 24065994..9ca5a434 100644
--- a/test/DefaultBtMessageDispatcherTest.cc
+++ b/test/DefaultBtMessageDispatcherTest.cc
@@ -138,7 +138,7 @@ public:
     option_.reset(new Option());
     option_->put(PREF_DIR, ".");
 
-    rg_.reset(new RequestGroup(option_));
+    rg_.reset(new RequestGroup(GroupId::create(), option_));
 
     dctx_.reset(new DownloadContext());
     bittorrent::load(A2_TEST_DIR"/test.torrent", dctx_, option_);
diff --git a/test/DefaultExtensionMessageFactoryTest.cc b/test/DefaultExtensionMessageFactoryTest.cc
index 10541878..4fbdf457 100644
--- a/test/DefaultExtensionMessageFactoryTest.cc
+++ b/test/DefaultExtensionMessageFactoryTest.cc
@@ -64,7 +64,7 @@ public:
     dctx_.reset(new DownloadContext());
 
     SharedHandle<Option> option(new Option());
-    requestGroup_.reset(new RequestGroup(option));
+    requestGroup_.reset(new RequestGroup(GroupId::create(), option));
     requestGroup_->setDownloadContext(dctx_);
 
     factory_.reset(new DefaultExtensionMessageFactory());
diff --git a/test/DownloadHandlerFactoryTest.cc b/test/DownloadHandlerFactoryTest.cc
index 42cf0af4..56dfcb16 100644
--- a/test/DownloadHandlerFactoryTest.cc
+++ b/test/DownloadHandlerFactoryTest.cc
@@ -51,7 +51,7 @@ CPPUNIT_TEST_SUITE_REGISTRATION( DownloadHandlerFactoryTest );
 void DownloadHandlerFactoryTest::testGetMetalinkPreDownloadHandler_extension()
 {
   SharedHandle<DownloadContext> dctx(new DownloadContext(0, 0, "test.metalink"));
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   SharedHandle<PreDownloadHandler> handler = DownloadHandlerFactory::getMetalinkPreDownloadHandler();
@@ -66,7 +66,7 @@ void DownloadHandlerFactoryTest::testGetMetalinkPreDownloadHandler_contentType()
 {
   SharedHandle<DownloadContext> dctx(new DownloadContext(0, 0, "test"));
   dctx->getFirstFileEntry()->setContentType("application/metalink+xml");
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   SharedHandle<PreDownloadHandler> handler = DownloadHandlerFactory::getMetalinkPreDownloadHandler();
@@ -85,7 +85,7 @@ void DownloadHandlerFactoryTest::testGetBtPreDownloadHandler_extension()
 {
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(0, 0, A2_TEST_DIR"/test.torrent"));
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   SharedHandle<PreDownloadHandler> handler = DownloadHandlerFactory::getBtPreDownloadHandler();
@@ -100,7 +100,7 @@ void DownloadHandlerFactoryTest::testGetBtPreDownloadHandler_contentType()
 {
   SharedHandle<DownloadContext> dctx(new DownloadContext(0, 0, "test"));
   dctx->getFirstFileEntry()->setContentType("application/x-bittorrent");
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   SharedHandle<PreDownloadHandler> handler = DownloadHandlerFactory::getBtPreDownloadHandler();
diff --git a/test/GroupIdTest.cc b/test/GroupIdTest.cc
new file mode 100644
index 00000000..ac26bc44
--- /dev/null
+++ b/test/GroupIdTest.cc
@@ -0,0 +1,127 @@
+#include "GroupId.h"
+
+#include <cppunit/extensions/HelperMacros.h>
+
+#include "TestUtil.h"
+#include "array_fun.h"
+
+namespace aria2 {
+
+class GroupIdTest:public CppUnit::TestFixture {
+
+  CPPUNIT_TEST_SUITE(GroupIdTest);
+  CPPUNIT_TEST(testCreate);
+  CPPUNIT_TEST(testToNumericId);
+  CPPUNIT_TEST(testExpandUnique);
+  CPPUNIT_TEST(testToHex);
+  CPPUNIT_TEST(testToAbbrevHex);
+  CPPUNIT_TEST_SUITE_END();
+public:
+  void setUp()
+  {
+    GroupId::clear();
+  }
+
+  void testCreate();
+  void testToNumericId();
+  void testExpandUnique();
+  void testToHex();
+  void testToAbbrevHex();
+};
+
+CPPUNIT_TEST_SUITE_REGISTRATION( GroupIdTest );
+
+void GroupIdTest::testCreate()
+{
+  SharedHandle<GroupId> gid = GroupId::create();
+  CPPUNIT_ASSERT(gid);
+  CPPUNIT_ASSERT(0 != gid->getNumericId());
+  CPPUNIT_ASSERT(!GroupId::import(gid->getNumericId()));
+  CPPUNIT_ASSERT(!GroupId::import(0));
+}
+
+void GroupIdTest::testToNumericId()
+{
+  a2_gid_t gid;
+  std::string hex;
+  hex = std::string(16, '0');
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::toNumericId(gid, hex.c_str()));
+
+  hex = std::string(16, 'f');
+  CPPUNIT_ASSERT_EQUAL(0, GroupId::toNumericId(gid, hex.c_str()));
+  CPPUNIT_ASSERT_EQUAL((a2_gid_t)UINT64_MAX, gid);
+
+  CPPUNIT_ASSERT_EQUAL(0, GroupId::toNumericId(gid, "1234567890abcdef"));
+  CPPUNIT_ASSERT_EQUAL((a2_gid_t)1311768467294899695LL, gid);
+
+  hex = std::string(15, 'f');
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::toNumericId(gid, hex.c_str()));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::toNumericId(gid, ""));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::toNumericId(gid, "1234567890abcdeg"));
+}
+
+void GroupIdTest::testExpandUnique()
+{
+  a2_gid_t gid;
+  SharedHandle<GroupId> ids[] = {
+    GroupId::import(0xff80000000010000LL),
+    GroupId::import(0xff80000000020001LL),
+    GroupId::import(0xfff8000000030000LL)
+  };
+  for(SharedHandle<GroupId>* i = vbegin(ids), *eoi = vend(ids); i != eoi;
+      ++i) {
+    CPPUNIT_ASSERT(*i);
+  }
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_NOT_UNIQUE,
+                       GroupId::expandUnique(gid, "ff8"));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::expandUnique(gid, "ffg"));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::expandUnique(gid,
+                                             std::string(17, 'a').c_str()));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_INVALID,
+                       GroupId::expandUnique(gid, ""));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_NOT_UNIQUE,
+                       GroupId::expandUnique(gid, "ff800000000"));
+
+  CPPUNIT_ASSERT_EQUAL(0, GroupId::expandUnique(gid, "ff8000000001"));
+  CPPUNIT_ASSERT_EQUAL(std::string("ff80000000010000"), GroupId::toHex(gid));
+
+  CPPUNIT_ASSERT_EQUAL(0, GroupId::expandUnique(gid, "ff8000000002"));
+  CPPUNIT_ASSERT_EQUAL(std::string("ff80000000020001"), GroupId::toHex(gid));
+
+  CPPUNIT_ASSERT_EQUAL(0, GroupId::expandUnique(gid, "fff800000003"));
+  CPPUNIT_ASSERT_EQUAL(std::string("fff8000000030000"), GroupId::toHex(gid));
+
+  CPPUNIT_ASSERT_EQUAL((int)GroupId::ERR_NOT_FOUND,
+                       GroupId::expandUnique(gid, "ff80000000031"));
+}
+
+void GroupIdTest::testToHex()
+{
+  CPPUNIT_ASSERT_EQUAL(std::string("1234567890abcdef"),
+                       GroupId::toHex(1311768467294899695LL));
+  CPPUNIT_ASSERT_EQUAL(std::string("0000000000000001"),
+                       GroupId::toHex(0000000000000000001LL));
+}
+
+void GroupIdTest::testToAbbrevHex()
+{
+  CPPUNIT_ASSERT_EQUAL(std::string("123456"),
+                       GroupId::toAbbrevHex(1311768467294899695LL));
+  CPPUNIT_ASSERT_EQUAL(std::string("000000"),
+                       GroupId::toAbbrevHex(0000000000000000001LL));
+}
+
+} // namespace aria2
diff --git a/test/HandshakeExtensionMessageTest.cc b/test/HandshakeExtensionMessageTest.cc
index 03c9cb72..26e52052 100644
--- a/test/HandshakeExtensionMessageTest.cc
+++ b/test/HandshakeExtensionMessageTest.cc
@@ -94,7 +94,7 @@ void HandshakeExtensionMessageTest::testDoReceivedAction()
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(METADATA_PIECE_SIZE, 0));
   SharedHandle<Option> op(new Option());
-  RequestGroup rg(op);
+  RequestGroup rg(GroupId::create(), op);
   rg.setDownloadContext(dctx);
 
   SharedHandle<TorrentAttribute> attrs(new TorrentAttribute());
diff --git a/test/Makefile.am b/test/Makefile.am
index 0a69c791..d15523fe 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -87,7 +87,8 @@ aria2c_SOURCES = AllTest.cc\
 	AbstractCommandTest.cc\
 	SinkStreamFilterTest.cc\
 	WrDiskCacheTest.cc\
-	WrDiskCacheEntryTest.cc
+	WrDiskCacheEntryTest.cc\
+	GroupIdTest.cc
 
 if ENABLE_XML_RPC
 aria2c_SOURCES += XmlRpcRequestParserControllerTest.cc
diff --git a/test/MetalinkPostDownloadHandlerTest.cc b/test/MetalinkPostDownloadHandlerTest.cc
index 8923dbda..1447b1bf 100644
--- a/test/MetalinkPostDownloadHandlerTest.cc
+++ b/test/MetalinkPostDownloadHandlerTest.cc
@@ -40,7 +40,7 @@ void MetalinkPostDownloadHandlerTest::testCanHandle_extension()
 {
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(0, 0, "test.metalink"));
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   MetalinkPostDownloadHandler handler;
@@ -55,7 +55,7 @@ void MetalinkPostDownloadHandlerTest::testCanHandle_contentType()
 {
   SharedHandle<DownloadContext> dctx(new DownloadContext(0, 0, "test"));
   dctx->getFirstFileEntry()->setContentType("application/metalink+xml");
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
 
   MetalinkPostDownloadHandler handler;
@@ -70,7 +70,7 @@ void MetalinkPostDownloadHandlerTest::testGetNextRequestGroups()
 {
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(1024, 0, A2_TEST_DIR"/test.xml"));
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
   rg.initPieceStorage();
   rg.getPieceStorage()->getDiskAdaptor()->enableReadOnly();
@@ -90,7 +90,7 @@ void MetalinkPostDownloadHandlerTest::testGetNextRequestGroups_withBaseUri()
   SharedHandle<DownloadContext> dctx
     (new DownloadContext(1024, 0, A2_TEST_DIR"/base_uri.xml"));
   dctx->getFirstFileEntry()->addUri("http://base/dir/base_uri.xml");
-  RequestGroup rg(option_);
+  RequestGroup rg(GroupId::create(), option_);
   rg.setDownloadContext(dctx);
   rg.initPieceStorage();
   rg.getPieceStorage()->getDiskAdaptor()->enableReadOnly();
diff --git a/test/RequestGroupManTest.cc b/test/RequestGroupManTest.cc
index 31fa8fbc..5c4c3e7e 100644
--- a/test/RequestGroupManTest.cc
+++ b/test/RequestGroupManTest.cc
@@ -33,7 +33,6 @@ private:
 public:
   void setUp()
   {
-    RequestGroup::resetGIDCounter();
     option_.reset(new Option());
   }
 
@@ -49,8 +48,10 @@ CPPUNIT_TEST_SUITE_REGISTRATION( RequestGroupManTest );
 
 void RequestGroupManTest::testIsSameFileBeingDownloaded()
 {
-  SharedHandle<RequestGroup> rg1(new RequestGroup(util::copy(option_)));
-  SharedHandle<RequestGroup> rg2(new RequestGroup(util::copy(option_)));
+  SharedHandle<RequestGroup> rg1(new RequestGroup(GroupId::create(),
+                                                  util::copy(option_)));
+  SharedHandle<RequestGroup> rg2(new RequestGroup(GroupId::create(),
+                                                  util::copy(option_)));
 
   SharedHandle<DownloadContext> dctx1
     (new DownloadContext(0, 0, "aria2.tar.bz2"));
@@ -118,51 +119,71 @@ void RequestGroupManTest::testLoadServerStat()
 void RequestGroupManTest::testChangeReservedGroupPosition()
 {
   SharedHandle<RequestGroup> gs[] = {
-    SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_))),
-    SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_))),
-    SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_))),
-    SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_)))
+    SharedHandle<RequestGroup>(new RequestGroup(GroupId::create(),
+                                                util::copy(option_))),
+    SharedHandle<RequestGroup>(new RequestGroup(GroupId::create(),
+                                                util::copy(option_))),
+    SharedHandle<RequestGroup>(new RequestGroup(GroupId::create(),
+                                                util::copy(option_))),
+    SharedHandle<RequestGroup>(new RequestGroup(GroupId::create(),
+                                                util::copy(option_)))
   };
   std::vector<SharedHandle<RequestGroup> > groups(vbegin(gs), vend(gs));
   RequestGroupMan rm(groups, 0, option_.get());
 
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)0, rm.changeReservedGroupPosition(1, 0, RequestGroupMan::POS_SET));
+    ((size_t)0, rm.changeReservedGroupPosition(gs[0]->getGID(),
+                                               0, RequestGroupMan::POS_SET));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)1, rm.changeReservedGroupPosition(1, 1, RequestGroupMan::POS_SET));
+    ((size_t)1, rm.changeReservedGroupPosition(gs[0]->getGID(),
+                                               1, RequestGroupMan::POS_SET));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)3, rm.changeReservedGroupPosition(1, 10,RequestGroupMan::POS_SET));
+    ((size_t)3, rm.changeReservedGroupPosition(gs[0]->getGID(),
+                                               10,RequestGroupMan::POS_SET));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)0, rm.changeReservedGroupPosition(1,-10,RequestGroupMan::POS_SET));
+    ((size_t)0, rm.changeReservedGroupPosition(gs[0]->getGID(),
+                                               -10, RequestGroupMan::POS_SET));
 
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)1, rm.changeReservedGroupPosition(2, 0, RequestGroupMan::POS_CUR));
+    ((size_t)1, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               0, RequestGroupMan::POS_CUR));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)2, rm.changeReservedGroupPosition(2, 1, RequestGroupMan::POS_CUR));
+    ((size_t)2, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               1, RequestGroupMan::POS_CUR));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)1, rm.changeReservedGroupPosition(2, -1,RequestGroupMan::POS_CUR));
+    ((size_t)1, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               -1,RequestGroupMan::POS_CUR));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)0, rm.changeReservedGroupPosition(2,-10,RequestGroupMan::POS_CUR));
+    ((size_t)0, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               -10, RequestGroupMan::POS_CUR));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)1, rm.changeReservedGroupPosition(2, 1, RequestGroupMan::POS_CUR));
+    ((size_t)1, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               1, RequestGroupMan::POS_CUR));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)3, rm.changeReservedGroupPosition(2, 10,RequestGroupMan::POS_CUR));
+    ((size_t)3, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               10, RequestGroupMan::POS_CUR));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)1, rm.changeReservedGroupPosition(2, -2,RequestGroupMan::POS_CUR));
+    ((size_t)1, rm.changeReservedGroupPosition(gs[1]->getGID(),
+                                               -2,RequestGroupMan::POS_CUR));
 
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)3, rm.changeReservedGroupPosition(4, 0, RequestGroupMan::POS_END));
+    ((size_t)3, rm.changeReservedGroupPosition(gs[3]->getGID(),
+                                               0, RequestGroupMan::POS_END));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)2, rm.changeReservedGroupPosition(4, -1,RequestGroupMan::POS_END));
+    ((size_t)2, rm.changeReservedGroupPosition(gs[3]->getGID(),
+                                               -1,RequestGroupMan::POS_END));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)0, rm.changeReservedGroupPosition(4,-10,RequestGroupMan::POS_END));
+    ((size_t)0, rm.changeReservedGroupPosition(gs[3]->getGID(),
+                                               -10, RequestGroupMan::POS_END));
   CPPUNIT_ASSERT_EQUAL
-    ((size_t)3, rm.changeReservedGroupPosition(4, 10,RequestGroupMan::POS_END));
+    ((size_t)3, rm.changeReservedGroupPosition(gs[3]->getGID(),
+                                               10, RequestGroupMan::POS_END));
 
   CPPUNIT_ASSERT_EQUAL((size_t)4, rm.getReservedGroups().size());
 
   try {
-    rm.changeReservedGroupPosition(5, 0, RequestGroupMan::POS_CUR);
+    rm.changeReservedGroupPosition(GroupId::create()->getNumericId(),
+                                   0, RequestGroupMan::POS_CUR);
     CPPUNIT_FAIL("exception must be thrown.");
   } catch(RecoverableException& e) {
     // success
diff --git a/test/RequestGroupTest.cc b/test/RequestGroupTest.cc
index c84ad280..7e6de0b5 100644
--- a/test/RequestGroupTest.cc
+++ b/test/RequestGroupTest.cc
@@ -36,7 +36,7 @@ void RequestGroupTest::testGetFirstFilePath()
   SharedHandle<DownloadContext> ctx
     (new DownloadContext(1024, 1024, "/tmp/myfile"));
 
-  RequestGroup group(option_);
+  RequestGroup group(GroupId::create(), option_);
   group.setDownloadContext(ctx);
 
   CPPUNIT_ASSERT_EQUAL(std::string("/tmp/myfile"), group.getFirstFilePath());
@@ -50,7 +50,7 @@ void RequestGroupTest::testCreateDownloadResult()
 {
   SharedHandle<DownloadContext> ctx
     (new DownloadContext(1024, 1024*1024, "/tmp/myfile"));
-  RequestGroup group(option_);
+  RequestGroup group(GroupId::create(), option_);
   group.setDownloadContext(ctx);
   group.initPieceStorage();
   {
diff --git a/test/RpcMethodTest.cc b/test/RpcMethodTest.cc
index a095993a..ae016d74 100644
--- a/test/RpcMethodTest.cc
+++ b/test/RpcMethodTest.cc
@@ -83,7 +83,6 @@ private:
 public:
   void setUp()
   {
-    RequestGroup::resetGIDCounter();
     option_.reset(new Option());
     option_->put(PREF_DIR, A2_TEST_OUT_DIR"/aria2_RpcMethodTest");
     option_->put(PREF_PIECE_LENGTH, "1048576");
@@ -176,8 +175,11 @@ void RpcMethodTest::testAddUri()
   {
     RpcResponse res = m.execute(req, e_.get());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
+    a2_gid_t gid;
+    CPPUNIT_ASSERT_EQUAL(0, GroupId::toNumericId
+                         (gid, downcast<String>(res.param)->s().c_str()));
     CPPUNIT_ASSERT_EQUAL(std::string("/sink"),
-                         e_->getRequestGroupMan()->findReservedGroup(2)->
+                         e_->getRequestGroupMan()->findReservedGroup(gid)->
                          getOption()->get(PREF_DIR));
   }
 }
@@ -271,7 +273,8 @@ void RpcMethodTest::testAddTorrent()
       (!File(e_->getOption()->get(PREF_DIR)+
              "/0a3893293e27ac0490424c06de4d09242215f0a6.torrent").exists());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
-    CPPUNIT_ASSERT_EQUAL(std::string("1"), downcast<String>(res.param)->s());
+    CPPUNIT_ASSERT_EQUAL(sizeof(a2_gid_t)*2,
+                         downcast<String>(res.param)->s().size());
   }
   e_->getOption()->put(PREF_RPC_SAVE_UPLOAD_METADATA, A2_V_TRUE);
   {
@@ -280,10 +283,12 @@ void RpcMethodTest::testAddTorrent()
       (File(e_->getOption()->get(PREF_DIR)+
             "/0a3893293e27ac0490424c06de4d09242215f0a6.torrent").exists());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
-    CPPUNIT_ASSERT_EQUAL(std::string("2"), downcast<String>(res.param)->s());
+    a2_gid_t gid;
+    CPPUNIT_ASSERT_EQUAL(0, GroupId::toNumericId
+                         (gid, downcast<String>(res.param)->s().c_str()));
 
     SharedHandle<RequestGroup> group =
-      e_->getRequestGroupMan()->findReservedGroup(1);
+      e_->getRequestGroupMan()->findReservedGroup(gid);
     CPPUNIT_ASSERT(group);
     CPPUNIT_ASSERT_EQUAL(e_->getOption()->get(PREF_DIR)+"/aria2-0.8.2.tar.bz2",
                          group->getFirstFilePath());
@@ -304,9 +309,12 @@ void RpcMethodTest::testAddTorrent()
   {
     RpcResponse res = m.execute(req, e_.get());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
+    a2_gid_t gid;
+    CPPUNIT_ASSERT_EQUAL(0, GroupId::toNumericId
+                         (gid, downcast<String>(res.param)->s().c_str()));
     CPPUNIT_ASSERT_EQUAL
       (dir+"/aria2-0.8.2.tar.bz2",
-       e_->getRequestGroupMan()->findReservedGroup(3)->getFirstFilePath());
+       e_->getRequestGroupMan()->findReservedGroup(gid)->getFirstFilePath());
     CPPUNIT_ASSERT
       (File(dir+"/0a3893293e27ac0490424c06de4d09242215f0a6.torrent").exists());
   }
@@ -367,8 +375,13 @@ void RpcMethodTest::testAddMetalink()
     CPPUNIT_ASSERT_EQUAL(0, res.code);
     const List* resParams = downcast<List>(res.param);
     CPPUNIT_ASSERT_EQUAL((size_t)2, resParams->size());
-    CPPUNIT_ASSERT_EQUAL(std::string("1"), downcast<String>(resParams->get(0))->s());
-    CPPUNIT_ASSERT_EQUAL(std::string("2"), downcast<String>(resParams->get(1))->s());
+    a2_gid_t gid1, gid2;
+    CPPUNIT_ASSERT_EQUAL
+      (0, GroupId::toNumericId
+       (gid1, downcast<String>(resParams->get(0))->s().c_str()));
+    CPPUNIT_ASSERT_EQUAL
+      (0, GroupId::toNumericId
+       (gid2, downcast<String>(resParams->get(1))->s().c_str()));
     CPPUNIT_ASSERT
       (!File(e_->getOption()->get(PREF_DIR)+
              "/c908634fbc257fd56f0114912c2772aeeb4064f4.meta4").exists());
@@ -379,8 +392,13 @@ void RpcMethodTest::testAddMetalink()
     CPPUNIT_ASSERT_EQUAL(0, res.code);
     const List* resParams = downcast<List>(res.param);
     CPPUNIT_ASSERT_EQUAL((size_t)2, resParams->size());
-    CPPUNIT_ASSERT_EQUAL(std::string("3"), downcast<String>(resParams->get(0))->s());
-    CPPUNIT_ASSERT_EQUAL(std::string("4"), downcast<String>(resParams->get(1))->s());
+    a2_gid_t gid3, gid4;
+    CPPUNIT_ASSERT_EQUAL
+      (0, GroupId::toNumericId
+       (gid3, downcast<String>(resParams->get(0))->s().c_str()));
+    CPPUNIT_ASSERT_EQUAL
+      (0, GroupId::toNumericId
+       (gid4, downcast<String>(resParams->get(1))->s().c_str()));
 #ifdef ENABLE_MESSAGE_DIGEST
     CPPUNIT_ASSERT
       (File(e_->getOption()->get(PREF_DIR)+
@@ -388,12 +406,12 @@ void RpcMethodTest::testAddMetalink()
 #endif // ENABLE_MESSAGE_DIGEST
 
     SharedHandle<RequestGroup> tar =
-      e_->getRequestGroupMan()->findReservedGroup(1);
+      e_->getRequestGroupMan()->findReservedGroup(gid3);
     CPPUNIT_ASSERT(tar);
     CPPUNIT_ASSERT_EQUAL(e_->getOption()->get(PREF_DIR)+"/aria2-5.0.0.tar.bz2",
                          tar->getFirstFilePath());
     SharedHandle<RequestGroup> deb =
-      e_->getRequestGroupMan()->findReservedGroup(2);
+      e_->getRequestGroupMan()->findReservedGroup(gid4);
     CPPUNIT_ASSERT(deb);
     CPPUNIT_ASSERT_EQUAL(e_->getOption()->get(PREF_DIR)+"/aria2-5.0.0.deb",
                          deb->getFirstFilePath());
@@ -408,8 +426,14 @@ void RpcMethodTest::testAddMetalink()
   {
     RpcResponse res = m.execute(req, e_.get());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
+    const List* resParams = downcast<List>(res.param);
+    CPPUNIT_ASSERT_EQUAL((size_t)2, resParams->size());
+    a2_gid_t gid5;
+    CPPUNIT_ASSERT_EQUAL
+      (0, GroupId::toNumericId
+       (gid5, downcast<String>(resParams->get(0))->s().c_str()));
     CPPUNIT_ASSERT_EQUAL(dir+"/aria2-5.0.0.tar.bz2",
-                         e_->getRequestGroupMan()->findReservedGroup(5)->
+                         e_->getRequestGroupMan()->findReservedGroup(gid5)->
                          getFirstFilePath());
 #ifdef ENABLE_MESSAGE_DIGEST
     CPPUNIT_ASSERT
@@ -462,12 +486,13 @@ void RpcMethodTest::testAddMetalink_withPosition()
 
 void RpcMethodTest::testChangeOption()
 {
-  SharedHandle<RequestGroup> group(new RequestGroup(option_));
+  SharedHandle<RequestGroup> group(new RequestGroup(GroupId::create(),
+                                                    option_));
   e_->getRequestGroupMan()->addReservedGroup(group);
 
   ChangeOptionRpcMethod m;
   RpcRequest req(ChangeOptionRpcMethod::getMethodName(), List::g());
-  req.params->append("1");
+  req.params->append(GroupId::toHex(group->getGID()));
   SharedHandle<Dict> opt = Dict::g();
   opt->put(PREF_MAX_DOWNLOAD_LIMIT->k, "100K");
 #ifdef ENABLE_BITTORRENT
@@ -505,12 +530,13 @@ void RpcMethodTest::testChangeOption()
 
 void RpcMethodTest::testChangeOption_withBadOption()
 {
-  SharedHandle<RequestGroup> group(new RequestGroup(option_));
+  SharedHandle<RequestGroup> group(new RequestGroup(GroupId::create(),
+                                                    option_));
   e_->getRequestGroupMan()->addReservedGroup(group);
 
   ChangeOptionRpcMethod m;
   RpcRequest req(ChangeOptionRpcMethod::getMethodName(), List::g());
-  req.params->append("1");
+  req.params->append(GroupId::toHex(group->getGID()));
   SharedHandle<Dict> opt = Dict::g();
   opt->put(PREF_MAX_DOWNLOAD_LIMIT->k, "badvalue");
   req.params->append(opt);
@@ -520,12 +546,13 @@ void RpcMethodTest::testChangeOption_withBadOption()
 
 void RpcMethodTest::testChangeOption_withNotAllowedOption()
 {
-  SharedHandle<RequestGroup> group(new RequestGroup(option_));
+  SharedHandle<RequestGroup> group(new RequestGroup(GroupId::create(),
+                                                    option_));
   e_->getRequestGroupMan()->addReservedGroup(group);
 
   ChangeOptionRpcMethod m;
   RpcRequest req(ChangeOptionRpcMethod::getMethodName(), List::g());
-  req.params->append("1");
+  req.params->append(GroupId::toHex(group->getGID()));
   SharedHandle<Dict> opt = Dict::g();
   opt->put(PREF_MAX_OVERALL_DOWNLOAD_LIMIT->k, "100K");
   req.params->append(opt);
@@ -648,7 +675,7 @@ void RpcMethodTest::testTellWaiting()
 #else // !ENABLE_BITTORRENT
   addUri("http://4/", e_);
 #endif // !ENABLE_BITTORRENT
-
+  const SharedHandle<RequestGroupMan>& rgman = e_->getRequestGroupMan();
   TellWaitingRpcMethod m;
   RpcRequest req(TellWaitingRpcMethod::getMethodName(), List::g());
   req.params->append(Integer::g(1));
@@ -657,9 +684,9 @@ void RpcMethodTest::testTellWaiting()
   CPPUNIT_ASSERT_EQUAL(0, res.code);
   const List* resParams = downcast<List>(res.param);
   CPPUNIT_ASSERT_EQUAL((size_t)2, resParams->size());
-  CPPUNIT_ASSERT_EQUAL(std::string("2"),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(rgman->getReservedGroups()[1]->getGID()),
                        getString(downcast<Dict>(resParams->get(0)), "gid"));
-  CPPUNIT_ASSERT_EQUAL(std::string("3"),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(rgman->getReservedGroups()[2]->getGID()),
                        getString(downcast<Dict>(resParams->get(1)), "gid"));
   // waiting.size() == offset+num
   req = RpcRequest(TellWaitingRpcMethod::getMethodName(), List::g());
@@ -715,9 +742,9 @@ void RpcMethodTest::testTellWaiting()
   CPPUNIT_ASSERT_EQUAL(0, res.code);
   resParams = downcast<List>(res.param);
   CPPUNIT_ASSERT_EQUAL((size_t)2, resParams->size());
-  CPPUNIT_ASSERT_EQUAL(std::string("4"),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(rgman->getReservedGroups()[3]->getGID()),
                        getString(downcast<Dict>(resParams->get(0)), "gid"));
-  CPPUNIT_ASSERT_EQUAL(std::string("3"),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(rgman->getReservedGroups()[2]->getGID()),
                        getString(downcast<Dict>(resParams->get(1)), "gid"));
   // negative offset and size < num
   req.params->set(1, Integer::g(100));
@@ -774,7 +801,7 @@ void RpcMethodTest::testGatherStoppedDownload()
   followedBy.push_back(3);
   followedBy.push_back(4);
   SharedHandle<DownloadResult> d(new DownloadResult());
-  d->gid = 1;
+  d->gid = GroupId::create();
   d->fileEntries = fileEntries;
   d->inMemoryDownload = false;
   d->sessionDownloadLength = UINT64_MAX;
@@ -787,9 +814,11 @@ void RpcMethodTest::testGatherStoppedDownload()
   gatherStoppedDownload(entry, d, keys);
 
   const List* followedByRes = downcast<List>(entry->get("followedBy"));
-  CPPUNIT_ASSERT_EQUAL(std::string("3"), downcast<String>(followedByRes->get(0))->s());
-  CPPUNIT_ASSERT_EQUAL(std::string("4"), downcast<String>(followedByRes->get(1))->s());
-  CPPUNIT_ASSERT_EQUAL(std::string("2"),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(3),
+                       downcast<String>(followedByRes->get(0))->s());
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(4),
+                       downcast<String>(followedByRes->get(1))->s());
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(2),
                        downcast<String>(entry->get("belongsTo"))->s());
 
   keys.push_back("gid");
@@ -805,26 +834,30 @@ void RpcMethodTest::testGatherProgressCommon()
   SharedHandle<DownloadContext> dctx(new DownloadContext(0, 0,"aria2.tar.bz2"));
   std::string uris[] = { "http://localhost/aria2.tar.bz2" };
   dctx->getFirstFileEntry()->addUris(vbegin(uris), vend(uris));
-  SharedHandle<RequestGroup> group(new RequestGroup(util::copy(option_)));
+  SharedHandle<RequestGroup> group(new RequestGroup(GroupId::create(),
+                                                    util::copy(option_)));
   group->setDownloadContext(dctx);
   std::vector<SharedHandle<RequestGroup> > followedBy;
   for(int i = 0; i < 2; ++i) {
-    followedBy.push_back(SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_))));
+    followedBy.push_back(SharedHandle<RequestGroup>
+                         (new RequestGroup(GroupId::create(),
+                                           util::copy(option_))));
   }
 
   group->followedBy(followedBy.begin(), followedBy.end());
-  group->belongsTo(2);
+  SharedHandle<GroupId> parent = GroupId::create();
+  group->belongsTo(parent->getNumericId());
 
   SharedHandle<Dict> entry = Dict::g();
   std::vector<std::string> keys;
   gatherProgressCommon(entry, group, keys);
 
   const List* followedByRes = downcast<List>(entry->get("followedBy"));
-  CPPUNIT_ASSERT_EQUAL(util::itos(followedBy[0]->getGID()),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(followedBy[0]->getGID()),
                        downcast<String>(followedByRes->get(0))->s());
-  CPPUNIT_ASSERT_EQUAL(util::itos(followedBy[1]->getGID()),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(followedBy[1]->getGID()),
                        downcast<String>(followedByRes->get(1))->s());
-  CPPUNIT_ASSERT_EQUAL(std::string("2"),
+  CPPUNIT_ASSERT_EQUAL(parent->toHex(),
                        downcast<String>(entry->get("belongsTo"))->s());
   const List* files = downcast<List>(entry->get("files"));
   CPPUNIT_ASSERT_EQUAL((size_t)1, files->size());
@@ -897,20 +930,23 @@ void RpcMethodTest::testGatherBitTorrentMetadata()
 void RpcMethodTest::testChangePosition()
 {
   e_->getRequestGroupMan()->addReservedGroup
-    (SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_))));
+    (SharedHandle<RequestGroup>(new RequestGroup(GroupId::create(),
+                                                 util::copy(option_))));
   e_->getRequestGroupMan()->addReservedGroup
-    (SharedHandle<RequestGroup>(new RequestGroup(util::copy(option_))));
+    (SharedHandle<RequestGroup>(new RequestGroup(GroupId::create(),
+                                                 util::copy(option_))));
 
+  a2_gid_t gid = e_->getRequestGroupMan()->getReservedGroups()[0]->getGID();
   ChangePositionRpcMethod m;
   RpcRequest req(ChangePositionRpcMethod::getMethodName(), List::g());
-  req.params->append("1");
+  req.params->append(GroupId::toHex(gid));
   req.params->append(Integer::g(1));
   req.params->append("POS_SET");
   RpcResponse res = m.execute(req, e_.get());
   CPPUNIT_ASSERT_EQUAL(0, res.code);
   CPPUNIT_ASSERT_EQUAL((int64_t)1, downcast<Integer>(res.param)->i());
   CPPUNIT_ASSERT_EQUAL
-    ((a2_gid_t)1, e_->getRequestGroupMan()->getReservedGroups()[1]->getGID());
+    (gid, e_->getRequestGroupMan()->getReservedGroups()[1]->getGID());
 }
 
 void RpcMethodTest::testChangePosition_fail()
@@ -937,13 +973,14 @@ void RpcMethodTest::testChangeUri()
   files[1]->addUri("http://example.org/mustremove2");
   SharedHandle<DownloadContext> dctx(new DownloadContext());
   dctx->setFileEntries(&files[0], &files[3]);
-  SharedHandle<RequestGroup> group(new RequestGroup(option_));
+  SharedHandle<RequestGroup> group(new RequestGroup(GroupId::create(),
+                                                    option_));
   group->setDownloadContext(dctx);
   e_->getRequestGroupMan()->addReservedGroup(group);
 
   ChangeUriRpcMethod m;
   RpcRequest req(ChangeUriRpcMethod::getMethodName(), List::g());
-  req.params->append("1"); // GID
+  req.params->append(GroupId::toHex(group->getGID())); // GID
   req.params->append(Integer::g(2)); // index of FileEntry
   SharedHandle<List> removeuris = List::g();
   removeuris->append("http://example.org/mustremove1");
@@ -1007,13 +1044,14 @@ void RpcMethodTest::testChangeUri_fail()
   }
   SharedHandle<DownloadContext> dctx(new DownloadContext());
   dctx->setFileEntries(&files[0], &files[3]);
-  SharedHandle<RequestGroup> group(new RequestGroup(option_));
+  SharedHandle<RequestGroup> group(new RequestGroup(GroupId::create(),
+                                                    option_));
   group->setDownloadContext(dctx);
   e_->getRequestGroupMan()->addReservedGroup(group);
 
   ChangeUriRpcMethod m;
   RpcRequest req(ChangeUriRpcMethod::getMethodName(), List::g());
-  req.params->append("1"); // GID
+  req.params->append(GroupId::toHex(group->getGID())); // GID
   req.params->append(Integer::g(1)); // index of FileEntry
   SharedHandle<List> removeuris = List::g();
   req.params->append(removeuris);
@@ -1028,12 +1066,12 @@ void RpcMethodTest::testChangeUri_fail()
   CPPUNIT_ASSERT_EQUAL(1, res.code);
 
   req.params->set(1, Integer::g(1));
-  req.params->set(0, String::g("2"));
+  req.params->set(0, String::g(GroupId::create()->toHex()));
   res = m.execute(req, e_.get());
-  // RPC request fails because GID#2 does not exist.
+  // RPC request fails because the given GID does not exist.
   CPPUNIT_ASSERT_EQUAL(1, res.code);
 
-  req.params->set(0, String::g("1"));
+  req.params->set(0, String::g(GroupId::toHex(group->getGID())));
   req.params->set(1, Integer::g(4));
   res = m.execute(req, e_.get());
   // RPC request fails because FileEntry#3 does not exist.
@@ -1083,7 +1121,7 @@ void RpcMethodTest::testPause()
   {
     PauseRpcMethod m;
     RpcRequest req(PauseRpcMethod::getMethodName(), List::g());
-    req.params->append("1");
+    req.params->append(GroupId::toHex(groups[0]->getGID()));
     RpcResponse res = m.execute(req, e_.get());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
   }
@@ -1091,7 +1129,7 @@ void RpcMethodTest::testPause()
   {
     UnpauseRpcMethod m;
     RpcRequest req(UnpauseRpcMethod::getMethodName(), List::g());
-    req.params->append("1");
+    req.params->append(GroupId::toHex(groups[0]->getGID()));
     RpcResponse res = m.execute(req, e_.get());
     CPPUNIT_ASSERT_EQUAL(0, res.code);
   }
@@ -1172,9 +1210,10 @@ void RpcMethodTest::testSystemMulticall()
   CPPUNIT_ASSERT_EQUAL(0, res.code);
   const List* resParams = downcast<List>(res.param);
   CPPUNIT_ASSERT_EQUAL((size_t)7, resParams->size());
-  CPPUNIT_ASSERT_EQUAL(std::string("1"),
+  SharedHandle<RequestGroupMan> rgman = e_->getRequestGroupMan();
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(rgman->getReservedGroups()[0]->getGID()),
                        downcast<String>(downcast<List>(resParams->get(0))->get(0))->s());
-  CPPUNIT_ASSERT_EQUAL(std::string("2"),
+  CPPUNIT_ASSERT_EQUAL(GroupId::toHex(rgman->getReservedGroups()[1]->getGID()),
                        downcast<String>(downcast<List>(resParams->get(1))->get(0))->s());
   CPPUNIT_ASSERT_EQUAL((int64_t)1,
                        downcast<Integer>
diff --git a/test/UTMetadataPostDownloadHandlerTest.cc b/test/UTMetadataPostDownloadHandlerTest.cc
index 16431f68..23966aa5 100644
--- a/test/UTMetadataPostDownloadHandlerTest.cc
+++ b/test/UTMetadataPostDownloadHandlerTest.cc
@@ -35,7 +35,7 @@ public:
     option_.reset(new Option());
     option_->put(PREF_DIR, A2_TEST_OUT_DIR);
     dctx_.reset(new DownloadContext(0, 0, A2_TEST_OUT_DIR"/something"));
-    requestGroup_.reset(new RequestGroup(option_));
+    requestGroup_.reset(new RequestGroup(GroupId::create(), option_));
     requestGroup_->setDownloadContext(dctx_);
   }