A substrate polishing apparatus is used to polish a surface of a substrate such as a semiconductor wafer to a flat mirror finish. The substrate polishing apparatus has a polishing table and a polishing pad mounted on the polishing table for polishing a semiconductor substrate. The polishing pad has a through hole formed therein. The substrate polishing apparatus also has a light emission and reception device for emitting measurement light through the through hole formed in the polishing pad to the semiconductor substrate and receiving reflected light from the semiconductor substrate so as to measure a film on the semiconductor substrate. The light emission and reception device is disposed in the polishing table. The substrate polishing apparatus includes a supply passage for supplying a fluid to a path of the measurement light. The supply passage has an outlet portion detachably mounted on the polishing table. The substrate polishing apparatus also includes a protection cover mounted on the polishing table and fitted into the through hole when the polishing pad is attached to the polishing table.