diff --git a/kenall.go b/kenall.go
index bc303f5..efd0d09 100644
--- a/kenall.go
+++ b/kenall.go
@@ -41,8 +41,18 @@ type (
 
 		token string
 	}
-	// A Option provides a Functional Option Pattern for kenall.Client.
-	Option func(*Client)
+
+	// A ClientOption provides a customize option for kenall.Client.
+	ClientOption interface {
+		Apply(*Client)
+	}
+	withHTTPClient struct {
+		client *http.Client
+	}
+	withEndpoint struct {
+		endpoint string
+	}
+
 	// A GetAddressResponse is a result from the kenall service of the API to get the address from the postal code.
 	GetAddressResponse struct {
 		Version   Version    `json:"version"`
@@ -82,7 +92,7 @@ type (
 )
 
 // NewClient creates kenall.Client with the authorization token provided by the kenall.
-func NewClient(token string, opts ...Option) (*Client, error) {
+func NewClient(token string, opts ...ClientOption) (*Client, error) {
 	if token == "" {
 		return nil, ErrInvalidArgument
 	}
@@ -94,24 +104,20 @@ func NewClient(token string, opts ...Option) (*Client, error) {
 	}
 
 	for _, opt := range opts {
-		opt(cli)
+		opt.Apply(cli)
 	}
 
 	return cli, nil
 }
 
 // WithHTTPClient injects optional HTTP Client to kenall.Client.
-func WithHTTPClient(cli *http.Client) Option {
-	return func(c *Client) {
-		c.HTTPClient = cli
-	}
+func WithHTTPClient(cli *http.Client) ClientOption {
+	return &withHTTPClient{client: cli}
 }
 
 // WithEndpoint injects optional endpoint to kenall.Client.
-func WithEndpoint(endpoint string) Option {
-	return func(c *Client) {
-		c.Endpoint = endpoint
-	}
+func WithEndpoint(endpoint string) ClientOption {
+	return &withEndpoint{endpoint: endpoint}
 }
 
 // GetAddress requests to the kenall service to get the address by postal code.
@@ -171,3 +177,13 @@ func (v *Version) UnmarshalJSON(data []byte) error {
 
 	return nil
 }
+
+// Apply implements kenall.ClientOption interface.
+func (w *withHTTPClient) Apply(cli *Client) {
+	cli.HTTPClient = w.client
+}
+
+// Apply implements kenall.ClientOption interface.
+func (w *withEndpoint) Apply(cli *Client) {
+	cli.Endpoint = w.endpoint
+}
diff --git a/kenall_test.go b/kenall_test.go
index 4cdf3c3..231d653 100644
--- a/kenall_test.go
+++ b/kenall_test.go
@@ -21,13 +21,14 @@ func TestNewClient(t *testing.T) {
 	t.Parallel()
 
 	cases := map[string]struct {
-		token string
-		opts  []kenall.Option
-		want  error
+		token      string
+		httpClient *http.Client
+		endpoint   string
+		want       error
 	}{
-		"Empty token":         {token: "", opts: nil, want: kenall.ErrInvalidArgument},
-		"Give token":          {token: "dummy", opts: nil, want: nil},
-		"Give token and opts": {token: "dummy", opts: []kenall.Option{kenall.WithEndpoint(""), kenall.WithHTTPClient(nil)}, want: nil},
+		"Empty token":         {token: "", httpClient: nil, endpoint: "", want: kenall.ErrInvalidArgument},
+		"Give token":          {token: "dummy", httpClient: nil, endpoint: "", want: nil},
+		"Give token and opts": {token: "dummy", httpClient: &http.Client{}, endpoint: "customize_endpoint", want: nil},
 	}
 
 	for name, c := range cases {
@@ -35,13 +36,24 @@ func TestNewClient(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			t.Parallel()
 
-			cli, err := kenall.NewClient(c.token, c.opts...)
+			opts := make([]kenall.ClientOption, 0, 2)
+			if c.httpClient != nil {
+				opts = append(opts, kenall.WithHTTPClient(c.httpClient))
+			}
+			if c.endpoint != "" {
+				opts = append(opts, kenall.WithEndpoint(c.endpoint))
+			}
+
+			cli, err := kenall.NewClient(c.token, opts...)
 			if !errors.Is(c.want, err) {
 				t.Errorf("give: %v, want: %v", err, c.want)
 			}
 
-			if len(c.opts) != 0 && cli.Endpoint != "" && cli.HTTPClient != nil {
-				t.Error("option is not reflected")
+			if c.httpClient != nil && cli.HTTPClient != c.httpClient {
+				t.Errorf("give: %v, want: %v", cli.HTTPClient, c.httpClient)
+			}
+			if c.endpoint != "" && cli.Endpoint != c.endpoint {
+				t.Errorf("give: %v, want: %v", cli.Endpoint, c.endpoint)
 			}
 		})
 	}