A projecting/receiving unit (52) projects a laser light to a peripheral portion (30) and receives the reflected light while a liquid is being fed to a substrate (14) and is flowing on the peripheral portion (30). A signal processing controller (54) processes the electric signal of the reflected light to decide the state of the peripheral portion (30). The state of the peripheral portion being polished is monitored. Moreover, the polish end point is detected. A transmission wave other than the laser light may also be used. The peripheral portion (30) may also be enclosed by a passage forming member thereby to form a passage properly. The peripheral portion can be properly measured even in the situation where the liquid is flowing on the substrate peripheral portion.