To obtain a high photocorrosion preventing effect by reducing a light illumination time on a wafer polishing surface to a necessary minimum level.

A photocorrosion suppressing device 36 comprises a detection means 37 for detecting a detection start time and a detection completion time for a polishing end point; a control means 16 for receiving a start signal of end point detection and a completion signal of end point detection; and a light illusion mode switching means 38 for switching a light illumination mode to a wafer polishing surface, on the basis of a command signal from the control means 16. The light illumination mode switching means 38 switches to a mode that stops light illumination on the wafer polishing surface at completion of the end point detection. The light illumination mode switching means 38 has a light-shielding plate 41, provided in between a light illumination end 40 and a polishing pad 13 and an air cylinder 44 to cause the light-shielding plate 41 to be moved toward and away from an irradiation optical path L.