Input.GetButtonUp

説明

buttonName によって識別される仮想ボタンを離したフレームの間だけ true を返します

You need to call this function from the Update function, since the state gets reset each frame. It will not return true until the user has pressed the button and released it again.

Use this only when implementing action like events IE: shooting a weapon.
Use Input.GetAxis for any kind of movement behaviour.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public GameObject projectile;
    void Update()
    {
        if (Input.GetButtonUp("Fire1"))
            Instantiate(projectile, transform.position, transform.rotation);
    }
}