Input.GetMouseButtonUp

説明

マウス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 mouse button and released it again. button values are 0 for left button, 1 for right button, 2 for the middle button.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Update() {
        if (Input.GetMouseButtonUp(0))
            Debug.Log("Pressed left click.");
        
        if (Input.GetMouseButtonUp(1))
            Debug.Log("Pressed right click.");
        
        if (Input.GetMouseButtonUp(2))
            Debug.Log("Pressed middle click.");
        
    }
}