A coupling mechanism which enables a rotating body to follow an undulation of a polishing surface without generating flutter or vibration of the rotating body, and can finely control a load on the rotating body on a polishing surface in a load range which is smaller than the gravity of rotating body is disclosed. The coupling mechanism includes an upper spherical bearing and a lower spherical bearing disposed between a drive shaft and the rotating body. The upper spherical bearing has a first concave contact surface and a second convex contact surface which are in contact with each other, and the lower spherical bearing has a third concave contact surface and a fourth convex contact surface which are in contact with each other. The first concave contact surface and the second convex contact surface are located above the third concave contact surface and the fourth convex contact surface. The first concave contact surface, the second convex contact surface, the third concave contact surface, the fourth convex contact surface are arranged concentrically.