An exposure apparatus projects a pattern of an original illuminated by an illumination system onto a substrate, by a projection optical system, to expose the substrate. A light-shielding member defines a position at which light falls on an image plane of the projection optical system. An illuminance sensor measures an illuminance on the image plane, and a controller detects a position, on the image plane, of the illuminance sensor. The controller detects the position of the illuminance sensor based on a peak of the output from the illuminance sensor and a position of the light shielding member obtained while moving the light-shielding member, so that a position at which light falls on the image plane moves along the image plane.