To simplify a mechanism to elevate an upper frame by a method wherein an air cylinder is used as a drive means to elevate an upper frame.

A maintenance space 15 is formed such that when polishing sheets 31 and 41 are exchanged, through drive of an air cylinder 50, an upper frame 11 is displaced to an upper release position, and mounting and demounting of polishing sheets 31B and 41B are practicable. A work to exchange the polishing sheets 31B and 41B is simplified by the maintenance space 15. After exchange, through drive of an air cylinder 50, the upper frame 11 is displaced to a lower shield position. Since an air cylinder 50 is used as an elevation drive means for the upper frame 11. When the upper frame is compared with a device employing a threadedly advancing mechanism of a male screw shaft and a female screw hole, structure is simplified.