A finishing grindstone member (20) is fixed to a spindle (10) by a bolt. In the spindle (10), there is formed a tool mounting portion (25) into which a rough grindstone member can be mounted. Into the tool mounting portion (25), there can be mounted not only the rough grindstone member but also a cover member (36). After a silicone wafer is roughly ground using the rough grindstone member, when finish grinding the silicone wafer using the finishing grindstone member (20), the cover member (36) is mounted onto the tool mounting portion (25) to thereby prevent the tool mounting portion (25) from being soiled.