To provide an illumination optical system capable of easily forming a desired light intensity distribution in which illumination non-uniformity is not distinctive at the pupil position of the illumination optical system or at a position conjugated with the pupil position, even when the light from a light source includes light intensity variations (non-uniformity) in the cross-section of light.

The illumination optical system for illuminating an irradiated plane M with illumination light provided from a light source 1 includes a spatial light modulator SM1, which is arranged in an optical path of the illumination optical system and forms the desired light intensity distribution at the pupil position of the illumination optical system or a position optically conjugated with the pupil position, and a diffuser 4, which is arranged at the illumination light incidence side of the spatial light modulator and diffuses the illumination light made incident on the spatial light modulator.