diff --git a/customers_test.go b/customers_test.go
index bf5f2bc..f4d9637 100644
--- a/customers_test.go
+++ b/customers_test.go
@@ -11,8 +11,9 @@ import (
 const testEndpoint = "https://api.test.fincode.jp/v1"
 
 func TestCustomers(t *testing.T) {
+	t.Parallel()
 	ctx := context.Background()
-	c, err := New(Endpoint(testEndpoint))
+	c, err := New(Endpoint(testEndpoint), WithHTTPClient(retryableHTTPClient(t)))
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/go.mod b/go.mod
index 8dd5a7d..8237e18 100644
--- a/go.mod
+++ b/go.mod
@@ -14,6 +14,7 @@ require (
 	go.opentelemetry.io/otel/metric v1.25.0
 	go.opentelemetry.io/otel/trace v1.25.0
 	go.uber.org/multierr v1.11.0
+	golang.org/x/sync v0.7.0
 )
 
 require (
@@ -142,7 +143,6 @@ require (
 	golang.org/x/exp v0.0.0-20240222234643-814bf88cf225 // indirect
 	golang.org/x/net v0.24.0 // indirect
 	golang.org/x/oauth2 v0.18.0 // indirect
-	golang.org/x/sync v0.7.0 // indirect
 	golang.org/x/sys v0.20.0 // indirect
 	golang.org/x/term v0.19.0 // indirect
 	golang.org/x/text v0.14.0 // indirect
diff --git a/option_test.go b/option_test.go
index 19d7a7b..6d37c8a 100644
--- a/option_test.go
+++ b/option_test.go
@@ -12,17 +12,9 @@ import (
 )
 
 func TestWithHTTPClient(t *testing.T) {
+	t.Parallel()
 	ctx := context.Background()
-	retryClient := retryablehttp.NewClient()
-	retryClient.RetryMax = 10
-	retryClient.Logger = nil
-	retryClient.CheckRetry = func(ctx context.Context, res *http.Response, err error) (bool, error) {
-		if res.StatusCode == 403 {
-			return true, nil
-		}
-		return false, nil
-	}
-	c, err := New(Endpoint(testEndpoint), WithHTTPClient(retryClient.StandardClient()))
+	c, err := New(Endpoint(testEndpoint), WithHTTPClient(retryableHTTPClient(t)))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -65,3 +57,17 @@ func TestWithHTTPClient(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func retryableHTTPClient(t *testing.T) *http.Client {
+	t.Helper()
+	retryClient := retryablehttp.NewClient()
+	retryClient.RetryMax = 10
+	retryClient.Logger = nil
+	retryClient.CheckRetry = func(ctx context.Context, res *http.Response, err error) (bool, error) {
+		if res.StatusCode == 403 {
+			return true, nil
+		}
+		return false, nil
+	}
+	return retryClient.StandardClient()
+}
diff --git a/payments_test.go b/payments_test.go
index 245dbbf..f60bda2 100644
--- a/payments_test.go
+++ b/payments_test.go
@@ -10,6 +10,7 @@ import (
 )
 
 func TestPayments(t *testing.T) {
+	t.Parallel()
 	ctx := context.Background()
 	ts := NewPaymentServer(t)
 	opts := []runn.Option{
@@ -41,7 +42,7 @@ func TestPayments(t *testing.T) {
 	t.Logf("customer_id: %s", customerID)
 	t.Logf("card_id: %s", cardID)
 
-	c, err := New(Endpoint(testEndpoint))
+	c, err := New(Endpoint(testEndpoint), WithHTTPClient(retryableHTTPClient(t)))
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/testutil_test.go b/testutil_test.go
index 8696335..32813d6 100644
--- a/testutil_test.go
+++ b/testutil_test.go
@@ -45,7 +45,7 @@ func NewPaymentServer(t *testing.T) *httptest.Server {
 	t.Helper()
 	ctx := context.Background()
 	faker := gofakeit.NewCrypto()
-	c, err := New(Endpoint(testEndpoint))
+	c, err := New(Endpoint(testEndpoint), WithHTTPClient(retryableHTTPClient(t)))
 	if err != nil {
 		t.Fatal(err)
 	}