To provide an illumination optical system capable of uniformly adjusting a pupil intensity distribution at each point on a surface to be irradiated.

This illumination optical system to illuminate the surface to be irradiated (M; W) with light from a light source 1 includes a distribution forming optical system 3, 4, 7, 8 having an optical integrator 8 and forming the pupil intensity distribution on an illumination pupil located at the backside of the optical integrator, and a transmission filter 9 having transmittance characteristics varying depending upon an angle of incidence of light, which is disposed in an illumination pupil space between an optical element having a power adjacent to the front of the illumination pupil and an optical element having a power adjacent to the backside of the illumination pupil and which is disposed at a position where light passing through only a part of the region of the illumination pupil or light having passed through only a part of the region of the illumination pupil enters.