The present invention relates to a method of cleaning an optical film-thickness measuring system used for measuring a film thickness of a substrate, such as a wafer, while polishing the substrate. The method includes: cleaning an optical sensor head (7) by supplying a rinsing liquid into a through-hole (51) formed in a polishing pad (2) on a polishing table (3) before or after polishing of a substrate (W) with use of slurry, the optical sensor head (7) being located below the through-hole (51); and discharging the rinsing liquid from the through-hole (51) through a drain line (54).