An illumination optical system which illuminates an illumination objective surface with a light from a light source. The illumination optical system includes a spatial light modulator which includes a plurality of optical elements arranged within a predetermined plane and controlled individually, and which forms a light intensity distribution in an illumination pupil of the illumination optical system; and a polarization unit which is arranged in a position optically conjugate with the predetermined plane, and which polarizes an incident light beam having a first and second partial light beams, coming into the polarization unit such that the first and second partial light beams have polarization states different from each other, and emits the polarized incident light beam as an outgoing light beam, wherein the polarization unit changes, in a cross section of the outgoing light beam, a ratio between a cross sectional areas of the first and second partial light beams.