To reduce the influence of a measurement light for monitoring a polishing state on a polished object.

During the polishing of a wafer 2, the measurement light emitted from a light source 21 is radiated to the wafer 2, and the spectral intensity of the reflected light is detected by a linear sensor 31. Based on the detection signals from the sensor 31, a signal processing part 11 monitors the polishing state of the wafer 2, and detects the termination point for the polishing of the wafer 2. A shutter mechanism control part 14 controls the motor 13b of a shutter mechanism 13 in correspondence to the polishing termination point detection signals from the signal processing part 11 to advance the light shielding member 13a into an optical path for the measurement light so as to shield the wafer 2 from the measurement light.