PURPOSE: To highly shorten the time required from polishing start to finish by providing an arithmetic part of an ultrasonic thickness gauge for continually calculating and determining the measurement value of the thickness of a stamper on the basis of the measurement signal of a sensor, and a control unit for stopping a polishing machine when the measurement value reaches a predetermined target value.

CONSTITUTION: An ultrasonic thickness gauge 3 or eddy-current thickness gauge consisting of a sensor 3a and an arithmetic part 3b continually measures the thickness of a stamper 1 through a protecting board 2 from the surface, opposite to the surface to which the stamper 1 is fixed, of the protecting board 2 without stopping the contact-sliding of an abrasive cloth 5 with the metal mold mounting surface 1a of the stamper 1. Then, a control unit 8 stops a polishing machine to terminate polishing, when the measurement value continually measured by the ultrasonic thickness gauge 3 or eddy-current thickness gauge reaches a predetermined target value.