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 and to reduce a damage when polishing the semiconductor wafer.

In a back pad 31 to serve as a backing material of a silicon wafer 19, the hardness of its peripheral part is made to be higher than that of a center part 32. Therefore, an uneven thickness of a silicon wafer 19 caused by a smashed peripheral part of the back pad 31 when polishing its surface can be prevented. The center part 32 of the back pad 31 absorbs an impact force during surface polishing, and thus, when polishing the silicon wafer, a damage can be comparatively reduced. In addition, a damage in a circumferential shape is never produced on a back face of the wafer.