To prevent the occurrence of an uneven thickness of a semiconductor wafer caused by a smashed peripheral part of a back pad when polishing its surface.

After storing a back pad 18 containing water in a water holding hole 18a in a guide ring 14 of a silicon wafer 19, a surface of the wafer 19 is polished by rotating an upper surface plate 15 on its axis and revolving it around on an lower surface plate 13 while supplying a polishing liquid. Since the pad 18 is made of a synthetic resin sheet with moderate hardness in comparison with conventional non woven fabric and ice frozen on the surface of the pad 18 becomes a cushion, the wafer 19 is by a moderate elastic holding force and a peripheral part of the pad 18 is kept in a good shape. Since the wafer 19 is held by the pad 18 so as to freely rotate, even if a composite force is concentrated on the peripheral part of the pad from the side of the upper surface plate 15, it is not easily smashed. As a result, the wafer 19 with a small uneven thickness can be obtained.