diff --git a/http3/roundtrip.go b/http3/roundtrip.go
index 53885e1e..eef93c28 100644
--- a/http3/roundtrip.go
+++ b/http3/roundtrip.go
@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"strings"
 	"sync"
+	"sync/atomic"
 
 	"golang.org/x/net/http/httpguts"
 
@@ -25,6 +26,11 @@ type roundTripCloser interface {
 	io.Closer
 }
 
+type roundTripCloserWithCount struct {
+	roundTripCloser
+	useCount atomic.Int64
+}
+
 // RoundTripper implements the http.RoundTripper interface
 type RoundTripper struct {
 	mutex sync.Mutex
@@ -82,7 +88,7 @@ type RoundTripper struct {
 	MaxResponseHeaderBytes int64
 
 	newClient func(hostname string, tlsConf *tls.Config, opts *roundTripperOpts, conf *quic.Config, dialer dialFunc) (roundTripCloser, error) // so we can mock it in tests
-	clients   map[string]roundTripCloser
+	clients   map[string]*roundTripCloserWithCount
 	udpConn   *net.UDPConn
 }
 
@@ -143,6 +149,7 @@ func (r *RoundTripper) RoundTripOpt(req *http.Request, opt RoundTripOpt) (*http.
 	if err != nil {
 		return nil, err
 	}
+	defer cl.useCount.Add(-1)
 	rsp, err := cl.RoundTripOpt(req, opt)
 	if err != nil {
 		r.removeClient(hostname)
@@ -160,12 +167,12 @@ func (r *RoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
 	return r.RoundTripOpt(req, RoundTripOpt{})
 }
 
-func (r *RoundTripper) getClient(hostname string, onlyCached bool) (rtc roundTripCloser, isReused bool, err error) {
+func (r *RoundTripper) getClient(hostname string, onlyCached bool) (rtc *roundTripCloserWithCount, isReused bool, err error) {
 	r.mutex.Lock()
 	defer r.mutex.Unlock()
 
 	if r.clients == nil {
-		r.clients = make(map[string]roundTripCloser)
+		r.clients = make(map[string]*roundTripCloserWithCount)
 	}
 
 	client, ok := r.clients[hostname]
@@ -188,7 +195,7 @@ func (r *RoundTripper) getClient(hostname string, onlyCached bool) (rtc roundTri
 			}
 			dial = r.makeDialer()
 		}
-		client, err = newCl(
+		c, err := newCl(
 			hostname,
 			r.TLSClientConfig,
 			&roundTripperOpts{
@@ -204,10 +211,12 @@ func (r *RoundTripper) getClient(hostname string, onlyCached bool) (rtc roundTri
 		if err != nil {
 			return nil, false, err
 		}
+		client = &roundTripCloserWithCount{roundTripCloser: c}
 		r.clients[hostname] = client
 	} else if client.HandshakeComplete() {
 		isReused = true
 	}
+	client.useCount.Add(1)
 	return client, isReused, nil
 }
 
@@ -276,3 +285,14 @@ func (r *RoundTripper) makeDialer() func(ctx context.Context, addr string, tlsCf
 		return quicDialer(ctx, r.udpConn, udpAddr, tlsCfg, cfg)
 	}
 }
+
+func (r *RoundTripper) CloseIdleConnections() {
+	r.mutex.Lock()
+	defer r.mutex.Unlock()
+	for hostname, client := range r.clients {
+		if client.useCount.Load() == 0 {
+			client.Close()
+			delete(r.clients, hostname)
+		}
+	}
+}
diff --git a/http3/roundtrip_test.go b/http3/roundtrip_test.go
index 14388b5f..8e859de5 100644
--- a/http3/roundtrip_test.go
+++ b/http3/roundtrip_test.go
@@ -8,6 +8,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"sync/atomic"
 	"time"
 
 	"github.com/quic-go/quic-go"
@@ -304,10 +305,10 @@ var _ = Describe("RoundTripper", func() {
 
 	Context("closing", func() {
 		It("closes", func() {
-			rt.clients = make(map[string]roundTripCloser)
+			rt.clients = make(map[string]*roundTripCloserWithCount)
 			cl := NewMockRoundTripCloser(mockCtrl)
 			cl.EXPECT().Close()
-			rt.clients["foo.bar"] = cl
+			rt.clients["foo.bar"] = &roundTripCloserWithCount{cl, atomic.Int64{}}
 			err := rt.Close()
 			Expect(err).ToNot(HaveOccurred())
 			Expect(len(rt.clients)).To(BeZero())
@@ -319,6 +320,53 @@ var _ = Describe("RoundTripper", func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(len(rt.clients)).To(BeZero())
 		})
+
+		It("closes idle connections", func() {
+			Expect(len(rt.clients)).To(Equal(0))
+			req1, err := http.NewRequest("GET", "https://site1.com", nil)
+			Expect(err).ToNot(HaveOccurred())
+			req2, err := http.NewRequest("GET", "https://site2.com", nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(req1.Host).ToNot(Equal(req2.Host))
+			ctx1, cancel1 := context.WithCancel(context.Background())
+			ctx2, cancel2 := context.WithCancel(context.Background())
+			req1 = req1.WithContext(ctx1)
+			req2 = req2.WithContext(ctx2)
+			roundTripCalled := make(chan struct{})
+			reqFinished := make(chan struct{})
+			rt.newClient = func(hostname string, tlsConf *tls.Config, opts *roundTripperOpts, conf *quic.Config, dialer dialFunc) (roundTripCloser, error) {
+				cl := NewMockRoundTripCloser(mockCtrl)
+				cl.EXPECT().Close()
+				cl.EXPECT().RoundTripOpt(gomock.Any(), gomock.Any()).DoAndReturn(func(r *http.Request, _ RoundTripOpt) (*http.Response, error) {
+					roundTripCalled <- struct{}{}
+					<-r.Context().Done()
+					return nil, nil
+				})
+				return cl, nil
+			}
+			go func() {
+				rt.RoundTrip(req1)
+				reqFinished <- struct{}{}
+			}()
+			go func() {
+				rt.RoundTrip(req2)
+				reqFinished <- struct{}{}
+			}()
+			<-roundTripCalled
+			<-roundTripCalled
+			// Both two requests are started.
+			Expect(len(rt.clients)).To(Equal(2))
+			cancel1()
+			<-reqFinished
+			// req1 is finished
+			rt.CloseIdleConnections()
+			Expect(len(rt.clients)).To(Equal(1))
+			cancel2()
+			<-reqFinished
+			// all requests are finished
+			rt.CloseIdleConnections()
+			Expect(len(rt.clients)).To(Equal(0))
+		})
 	})
 
 	Context("reusing udpconn", func() {