An object of the present invention is to provide a polishing apparatus with a grinding plate that can easily and reliably be installed on and detached from a turntable. The polishing apparatus has a grinding plate tool, fixedly mounted on the turntable, which includes the grinding plate, and a top ring for holding a workpiece to be polished and pressing the workpiece against the grinding plate in sliding contact therewith for polishing a surface of the workpiece to a flat, mirror finish. A clamping mechanism is mounted in the turntable for fixing an outer circumferential flange of the grinding plate tool to the turntable.