An exposure apparatus is provided. An illumination optical system in the apparatus includes a diffraction optical element, a condensing optical system a detector that detects a light beam that exited from the condensing optical system, and a first diaphragm that can be inserted/removed in/from a position near a predetermined plane in an optical path where the condensing optical system condenses a light beam. The first diaphragm has an opening diameter such that an output of the detector decreases when an incident angle of light from a light source on the diffraction optical element deviates from a target angle. Based on an output of the detector when the first diaphragm is inserted in the position and an output of the detector when the first diaphragm is retracted from the position, a controller performs a process of adjusting the incident angle.