In a first step, an output of an eddy current sensor is measured while a polishing target whose film thickness has been known is in contact with the polishing face, thereby obtaining a measurement value of the eddy current sensor which corresponds to the film thickness. In a second step, an output of the eddy current sensor is measured when the polishing target is polished while pressed against the polishing face, thereby obtaining a measurement value of the eddy current sensor that corresponds to a film thickness during polishing. A correspondence relationship between the film thickness of the polishing target and the measurement value of the eddy current sensor is determined from the measurement value obtained in the first step and the measurement value obtained in the second step.