A polishing apparatus capable of accurately measuring a film thickness by regulating a quantity of light illuminating a wafer is disclosed. The polishing apparatus includes: a light source; an illuminating fiber having distal ends arranged at different locations in the polishing table; and a light-receiving fiber having distal ends arranged at the different locations in the polishing table. The illuminating fiber includes a first illuminating fiber and a second illuminating fiber. A first dimmer is attached to the first illuminating fiber and the second illuminating fiber, and a second dimmer is attached to at least one of the first illuminating fiber and the second illuminating fiber.