An accuracy of detecting a slip out of a substrate from a top ring is improved. A polishing unit 300 includes a polishing table 350, a top ring 302, a light emitting member 371, a slip-out detector 370, and an elimination mechanism 380. A polishing pad 352 that polishes a substrate WF is attached to the polishing table 350. The top ring 302 holds the substrate WF to press the substrate WF against the polishing pad 352. The light emitting member 371 emits a light to a detection area 372 on the polishing pad 352. The slip-out detector 370 detects a slip out of the substrate WF from the top ring 302 based on the light reflected from the detection area 372. The elimination mechanism 380 eliminates a polishing liquid flowing into the detection area 372.