An exposure apparatus includes (a) a projection optical system to project a reticle pattern onto a plate by using a light from a light source, and (b) a photo detector unit to detect the light via the projection optical system. The photo detector unit includes (i) a substrate, which is patterned with a wiring pattern and transmits the light, (ii) a detector to detect the light, and (iii) a bump to space the substrate from the detector, and to electrically connect the detector and the wiring pattern of the substrate.