Input.GetKeyDown working smoothly

I want to make simple movement, but i have a problem. I have to click right arrow many times in order to move object. What should I do to make it work repeatedly when I simply hold the button?

    void Update () {
         if (Input.GetKeyDown(KeyCode.RightArrow)) {
             transform.Translate(new Vector3(movement_speed,0,0));
         }
     }