An exposure apparatus includes an image sensor, a measurement optical system configured to guide measurement light to obliquely enter the projection optical system, and further, to guide the measurement light returned from the projection optical system to the image sensor, and a control unit configured to calculate surface position information of the substrate based on the output from the image sensor. The control unit calculates the surface position information of the substrate based on an interval between the image, of a mark arranged on the original stage, formed by the measurement light, which has passed through the mark, and the image of the mark formed by the measurement light reflected by the mark.