An illumination optical system that illuminates a target surface includes a first deflector, a second deflector, and an optical integrator. The first deflector is arranged on a first face crossing an optical path of light from a light source and has a period in a first direction defined on the first face. The second deflector is arranged on a second face crossing an optical path of light from the first deflector and has a period in a second direction defined on the second face. The optical integrator has a plurality of wavefront division facets arrayed on a third face crossing an optical axis of light from the second deflector. At least one of the first and second deflectors rotates about an optical axis of the illumination optical system or about an axis parallel to the optical axis in order to adjust a pattern of an illumination distribution.