An illumination optical system for illuminating a target surface using light from a light source includes a modified illumination generator for generating a modified illumination with a predetermined polarization state for the target surface, wherein the modified illumination generator includes a .lamda./4 phase plate unit that includes a .lamda./4 phase plate for converting a circularly polarized light into a linearly polarized light in a predetermined direction, and a diffractive optical element unit that is arranged in a substantially conjugate relationship with the target surface, and includes a diffractive optical element used for the .lamda./4 phase plate to generate a predetermined illumination intensity distribution when the diffractive optical element unit receives the linearly polarized light.