After a hole is formed in a polishing pad, a transparent window plate is inserted into the hole. Here, a gap is left between the upper surface of the transparent window plate and the outermost surface constituting the working surface of the polishing pad. During polishing, the polishing head holding the wafer applies a load to the polishing pad by means of a load-applying mechanism, so that the polishing pad and transparent window plate are compressed. In this case, the system is arranged so that the gap remains constant, and so that a dimension equal to or greater than a standard value is maintained. Since the upper surface of the transparent window plate is recessed from the upper surface of the polishing pad, there is no scratching of the surface of the transparent window plate during dressing. Accordingly, the polishing pad has a long useful life.