An illumination optical system which illuminates a surface to be illuminated on the basis of light from a light source has a first optical path in which a diffractive optical element can be arranged at a first position thereof; a second optical path in which a spatial light modulator with a plurality of optical elements arrayed two-dimensionally and controlled individually can be arranged at a second position thereof; and a third optical path which is an optical path of light having passed via at least one of the first optical path and the second optical path and in which a distribution forming optical system is arranged. The distribution forming optical system forms a predetermined light intensity distribution on an illumination pupil located in the third optical path, based on the light having passed via at least one of the first and second optical paths.