diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..2d83068
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+coverage.out
diff --git a/.octocov.yml b/.octocov.yml
index 94276ec..3eb5749 100644
--- a/.octocov.yml
+++ b/.octocov.yml
@@ -1,11 +1,12 @@
-# generated by octocov init
 coverage:
   if: true
 codeToTestRatio:
   code:
     - '**/*.go'
+    - '!testutil/**/*.go'
     - '!**/*_test.go'
   test:
+    - 'testutil/**/*.go'
     - '**/*_test.go'
 testExecutionTime:
   if: true
diff --git a/Makefile b/Makefile
index 42dcbc3..ca6c15e 100644
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,9 @@ default: test
 ci: depsdev test
 
 test:
-	go test ./... -coverprofile=coverage.out -covermode=count
+	cp go.mod testdata/go_test.mod
+	go mod tidy -modfile=testdata/go_test.mod
+	go test ./... -modfile=testdata/go_test.mod -coverprofile=coverage.out -covermode=count
 
 lint:
 	golangci-lint run ./...
diff --git a/rl_test.go b/rl_test.go
new file mode 100644
index 0000000..a74bed9
--- /dev/null
+++ b/rl_test.go
@@ -0,0 +1,60 @@
+package rl_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/go-chi/httprate"
+	"github.com/k1LoW/rl"
+	"github.com/k1LoW/rl/testutil"
+)
+
+func TestRL(t *testing.T) {
+	tests := []struct {
+		name         string
+		keyFunc      httprate.KeyFunc
+		reqLimit     int
+		hosts        []string
+		wantReqCount int
+	}{
+		{"key by ip", httprate.KeyByIP, 10, []string{"a.example.com", "b.example.com"}, 10},
+		{"key by host", testutil.KeyByHost, 10, []string{"a.example.com", "b.example.com"}, 20},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			r := http.NewServeMux()
+			r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+				w.Write([]byte("Hello, world"))
+			})
+			l := testutil.NewLimiter(tt.reqLimit, tt.keyFunc)
+			m := rl.New(l)
+			ts := httptest.NewServer(m(r))
+			t.Cleanup(func() {
+				ts.Close()
+			})
+			got := 0
+		L:
+			for {
+				for _, host := range tt.hosts {
+					req, err := http.NewRequest("GET", ts.URL, nil)
+					if err != nil {
+						t.Fatal(err)
+					}
+					req.Host = host
+					res, err := http.DefaultClient.Do(req)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if res.StatusCode == http.StatusTooManyRequests {
+						break L
+					}
+					got++
+				}
+			}
+			if got != tt.wantReqCount {
+				t.Errorf("got %v want %v", got, tt.wantReqCount)
+			}
+		})
+	}
+}
diff --git a/testdata/go_test.mod b/testdata/go_test.mod
new file mode 100644
index 0000000..1db5631
--- /dev/null
+++ b/testdata/go_test.mod
@@ -0,0 +1,10 @@
+module github.com/k1LoW/rl
+
+go 1.21.0
+
+require (
+	github.com/go-chi/httprate v0.7.4
+	golang.org/x/sync v0.3.0
+)
+
+require github.com/cespare/xxhash/v2 v2.1.2 // indirect
diff --git a/testdata/go_test.sum b/testdata/go_test.sum
new file mode 100644
index 0000000..9231e1e
--- /dev/null
+++ b/testdata/go_test.sum
@@ -0,0 +1,6 @@
+github.com/cespare/xxhash/v2 v2.1.2 h1:YRXhKfTDauu4ajMg1TPgFO5jnlC2HCbmLXMcTG5cbYE=
+github.com/cespare/xxhash/v2 v2.1.2/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+github.com/go-chi/httprate v0.7.4 h1:a2GIjv8he9LRf3712zxxnRdckQCm7I8y8yQhkJ84V6M=
+github.com/go-chi/httprate v0.7.4/go.mod h1:6GOYBSwnpra4CQfAKXu8sQZg+nZ0M1g9QnyFvxrAB8A=
+golang.org/x/sync v0.3.0 h1:ftCYgMx6zT/asHUrPw8BLLscYtGznsLAnjq5RH9P66E=
+golang.org/x/sync v0.3.0/go.mod h1:FU7BRWz2tNW+3quACPkgCx/L+uEAv1htQ0V83Z9Rj+Y=
diff --git a/testutil/limiter.go b/testutil/limiter.go
new file mode 100644
index 0000000..b2fb30b
--- /dev/null
+++ b/testutil/limiter.go
@@ -0,0 +1,72 @@
+package testutil
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/go-chi/httprate"
+)
+
+type Limiter struct {
+	counts   map[string]map[time.Time]int
+	reqLimit int
+	keyFunc  httprate.KeyFunc
+}
+
+func KeyByHost(r *http.Request) (string, error) {
+	return r.Host, nil
+}
+
+func NewLimiter(reqLimit int, keyFunc httprate.KeyFunc) *Limiter {
+	return &Limiter{
+		counts:   map[string]map[time.Time]int{},
+		reqLimit: reqLimit,
+		keyFunc:  keyFunc,
+	}
+}
+
+func (l *Limiter) Name() string {
+	return "testutil.Limiter"
+}
+
+func (l *Limiter) KeyAndRateLimit(r *http.Request) (string, int, time.Duration, error) {
+	key, err := l.keyFunc(r)
+	if err != nil {
+		return "", 0, 0, err
+	}
+	return key, l.reqLimit, time.Second, nil
+}
+
+func (l *Limiter) SetXRateLimitHeaders(err error) bool {
+	return true
+}
+
+func (l *Limiter) OnRequestLimit() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusTooManyRequests)
+		w.Write([]byte("Too many requests"))
+	}
+}
+
+func (l *Limiter) Get(key string, window time.Time) (count int, err error) {
+	if _, ok := l.counts[key]; !ok {
+		l.counts[key] = map[time.Time]int{}
+		return 0, nil
+	}
+	if _, ok := l.counts[key][window]; !ok {
+		l.counts[key][window] = 0
+		return 0, nil
+	}
+	return l.counts[key][window], nil
+}
+
+func (l *Limiter) Increment(key string, currentWindow time.Time) error {
+	if _, ok := l.counts[key]; !ok {
+		l.counts[key] = map[time.Time]int{}
+	}
+	if _, ok := l.counts[key][currentWindow]; !ok {
+		l.counts[key][currentWindow] = 0
+	}
+	l.counts[key][currentWindow]++
+	return nil
+}