The answer of someone's question in android developer facebook group https://goo.gl/nA1Vnz

LogCat

01-06 01:24:42.095 11422-11422/com.akexorcist.myapplication E/Check: onResponse
01-06 01:24:42.095 11422-11422/com.akexorcist.myapplication E/Check: Message : มีปัญหาอะไร?

MainActivity.java

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ServiceConnection.getService().getData().enqueue(new Callback<ResponseData>() {
            @Override
            public void onResponse(Response<ResponseData> response, Retrofit retrofit) {
                Log.e("Check", "onResponse");
                Log.e("Check", "Message : " + response.body().getMessage());

            }

            @Override
            public void onFailure(Throwable t) {
                Log.e("Check", "onFailure");
            }
        });
    }
}

MyServiceInterface.java

public interface MyServiceInterface {
    @POST("your_target_url")
    Call<ResponseData> getData();
}

MyServiceManager.java

public class MyServiceManager {
    private static Retrofit retrofit;

    private static Retrofit getServiceInstance() {
        if(retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl("your_base_url")
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();
        }
        return retrofit;
    }

    public static MyServiceInterface getService() {
        return getServiceInstance().create(MyServiceInterface.class);
    }
}

ResponseData.java

public class ResponseData {
    String status;
    String message;

    public String getStatus() {
        return status;
    }

    public String getMessage() {
        return message;
    }
}

Server Response

status : 200
Connection : keep-alive
Content-Length : 63
Content-Type : application/json; charset=utf-8
Date : Tue, 05 Jan 2016 18:13:28 GMT
ETag : W/"3f-4db558be"
X-Powered-By : Express

{"status":"ok","message":"มีปัญหาอะไร?"}