To obtain a polishing surface provided with a high flat surface precision in polishing it by sucking a wafer through a backing pad.

A wafer 1 is sucked to a front surface of a top ring 11 through a backing pad 12. In a back surface of the top ring 11, a circular cavity part 18 concentric with the wafer 1 is formed. In the backing pad 1, plural through holes 16 are formed, and in the top ring 11, plural through holes 17 are formed. The through hole 16 is connected to the cavity part 18 through the through hole 17. This cavity part 18 is connected through a pressure rising passage 21 interior of a main shaft 15 to a vacuum pump 31, a pure water supply device 32, and a high pressure air supply device 33. In polishing the wafer 1, pressure of pure water in the cavity part 18 is adjusted, thereby uniformity of in-surface distribution of rigidity of the backing pad 12 is improved.