An illumination optical system for illuminating an irradiated plane M with illumination light provided from a light source includes a spatial light modulator, which is arranged in an optical path of the illumination optical system and forms a desired light intensity distribution at a pupil position of the illumination optical system or a position optically conjugated with the pupil position, and a diffuser, which is arranged at an incidence side of the spatial light modulator through which the illumination light enters.