Rotate around center of Object

Unity Engine
1

I have an Object and want to rotate it around center, but it really rotate about its pivot. Here is my code:
transform.Rotate(Vector3.right * (Input.mousePosition.y - lastMousePos.y), Space.World);
transform.Rotate(Vector3.down * (Input.mousePosition.x - lastMousePos.x), Space.World);
how can I rotate it around the center?

2