2D : Rotation of Object With x-y Axis

I am developing 2D shooting game in that I have plane game object with x axis on right and y axis on top position. Now on bottom side of the plane I have one object which I want to rotate in mouse position.

Here is my code:

function Update () {
    var mousePos = Input.mousePosition;
    mousePos.z = 20.0f; //The distance from the camera to the player object
    var lookPos : Vector3 = Camera.mainCamera.ScreenToWorldPoint(mousePos);
    lookPos = lookPos - mousePos;
    var angle : float = Mathf.Atan2(lookPos.y, lookPos.x) * Mathf.Rad2Deg;
    transform.rotation = Quaternion.AngleAxis(angle, Vector3.forward);
}

But here angle becomes so large . And it is rotating on x-y axis but not with mouse position. So what do i need to do? Please help me to solve my problem.

Thank you for your support and help.

Assuming this is attached to the object that is rotating, line 5 should be:

lookPos = lookPos - transform.position;

function Update ()
{
mouse_pos = Input.mousePosition;
mouse_pos.z = 5.23; //The distance between the camera and object
object_pos = Camera.main.WorldToScreenPoint(target.position);
mouse_pos.x = mouse_pos.x - object_pos.x;
mouse_pos.y = mouse_pos.y - object_pos.y;
angle = Mathf.Atan2(mouse_pos.y, mouse_pos.x) * Mathf.Rad2Deg;
transform.rotation = Quaternion.Euler(Vector3(0, 0, angle - 90));
}

I got my solution.