An exposure apparatus for exposing a substrate to light via a reticle. The apparatus includes a substrate stage configured to hold the substrate and to move, a projection optical system configured to project a pattern of the reticle onto the substrate, a flat plate having a surface whose height is substantially the same as that of the substrate held by the substrate stage and to move, a supply unit having a supply nozzle and configured to supply liquid through the supply nozzle to a gap formed between a final surface of the projection optical system and at least one of the substrate held by the substrate stage and the flat plate, a recovery unit having a recovery nozzle and configured to recover liquid filling the gap through the recovery nozzle, and a sensor arranged on the flat plate and configured to measure illuminance.