A polishing method capable of accurately measuring a film thickness of a substrate, such as a wafer, by enhancing light intensity of a flash-light source, such as a xenon flash lamp is disclosed. The polishing method includes: while an optical sensor head is moving across a substrate, causing a flash-light source to emit light plural times in a first exposure time of a light detector to direct the light to the substrate via the optical sensor head, capturing reflected light from the substrate by the light detector via the optical sensor head, further causing the flash-light source to emit light plural times in a second exposure time of the light detector to direct the light to the substrate via the optical sensor head, and capturing reflected light from the substrate by the light detector via the optical sensor head; generating a spectrum of the reflected light; and detecting a surface state of the substrate from the spectrum.