The present invention provides an illumination optical system which illuminates an illumination target surface with a light beam from a light source, the system including a plurality of adjustment units each having one of a reflectance distribution and a transmittance distribution to adjust an incident angle distribution of the light beam which impinges on the illumination target surface, the plurality of adjustment units including an adjustment unit which adjusts differences between light amounts, in a first direction, of incident angle distributions of light beams at a plurality of points on the illumination target surface, and light amounts, in a second direction, thereof, and an adjustment unit which adjusts at least one of light amount differences, in the first direction, of the incident angle distributions of the light beams at the plurality of points on the illumination target surface, and light amount differences, in the second direction, thereof.