A method of monitoring progress of polishing of a substrate having at least two regions including a first region and a second region with different structures is provided. The method includes: applying light to plural measurement points on the substrate during polishing of the substrate; receiving reflected light from each measurement point; measuring intensity of the reflected light; producing a spectrum of the reflected light from the intensity; classifying the spectrum as spectrum of the reflected light from the first region or as spectrum of the reflected light from the second region based on a shape of the spectrum or the intensity of the reflected light; and monitoring progress of polishing of the substrate based on a temporal change in the spectrum of the reflected light from the first region.