An upper polishing plate is moved downward until facing a lower polishing plate to polish a work piece. The upper polishing plate is rotated in a horizontal plane together with a first elastic member, a second elastic member, an outer member and a connecting member. A pressure difference between a first pressing force pressing the outer member or an inner member upward and a second pressing force pressing the outer member or the inner member downward, which is produced in a first closed space by supplying a compressed fluid into and discharging the same from the first closed space, is adjusted, so that a third pressing force of the upper polishing plate, which presses a work piece, can be adjusted.