A polishing apparatus detects the escape of a substrate from a top ring during polishing. The polishing apparatus includes a polishing table having a polishing pad, a top ring, and a substrate escape detection section for detecting escape of the substrate from the top ring. The detection section includes a light irradiation member for irradiating an area of the upper surface of the polishing pad with light, a controller for controlling the light irradiation of the light irradiation member, an image-taking member for taking an image of the area irradiated with the light, and an information processing member for processing information outputted from the image-taking member. The controller controls the light irradiation member in such a manner that it performs light irradiation at least for a period of time during which the substrate is regarded as being in contact with the polishing pad.