A foreign substance inspection apparatus includes a light projecting unit, a detector which detects the intensity of scattered light of light projected onto the surface of an object to be detected by the light projecting unit, in association with the two-dimensional coordinate position on the surface, and a processing unit. The relationship between the intensity of the scattered light detected by the detector and the particle size of the foreign substance differs depending on the two-dimensional coordinate position on the surface. The processing unit determines, a conversion curve to convert the intensity of the scattered light detected by the detector into the particle size of the foreign substance, in accordance with the two-dimensional coordinate position of the foreign substance detected by the detector, and converts the intensity of the scattered light detected by the detector into the particle size of the foreign substance using the determined conversion curve.