A polishing apparatus for polishing a substrate is provided. The polishing apparatus includes: a polishing table holding a polishing pad; a top ring configured to press the substrate against the polishing pad; first and second optical heads each configured to apply the light to the substrate and to receive reflected light from the substrate; spectroscopes each configured to measure at each wavelength an intensity of the reflected light received; and a processor configured to produce a spectrum indicating a relationship between intensity and wavelength of the reflected light. The first optical head is arranged so as to face a center of the substrate, and the second optical head is arranged so as to face a peripheral portion of the substrate.