To replace an O-ring without inserting the upper body or an arm into the under space of a lifted pressurizing plate or working upwardly from the downside, and thus to easily and safely exchange the O-ring.

This polishing jig 28 comprises a disk part 281, a cylindrical wall plate part 282 disposed around the disk part 281, and a plurality of annular grooves 20 formed concentrically to the disk part in the bottom parts of a recessed part 28a and a recessed part 28b formed with the disk part 281 and the cylindrical wall plate part 282. The polishing jig 28 in which O-rings 21 are selectively mounted to the annular grooves 20 is engaged under the pressurizing plate 8. A splash preventing skirt 41 is provided for preventing intrusion of polishing liquid, and can be turned up during working.