To polish a wafer into highly accurate flatness by making uniform the gap between the inside face of a retainer ring and the outer circumferential edge of a wafer.

A dummy 7 is inserted into a gap between the inside face 6a of a retainer ring 6 and a cutout part 1a of a wafer 1. The dummy 7 is made of Si or SiO similar to the wafer 1, or polytetrafluoroethylene resin of difficult polishing material. By inserting the dummy 7 into the gap between the inside face 6a and the cutout part 1a of the wafer 1, the gap between the inside face 6a and the outer circumferential edge of the wafer 1 becomes uniform even at the cutout part 1a, a polishing head 3 in the vicinity of the outer circumferential edge of the wafer 1 is prevented from swelling, and the swelling quantity can be reduced. Consequently, the wafer 1 can be polished into highly accurate flatness.