To provide an illumination optical system having illuminance uniform to a prescribed irradiation surface (exposure surface) and a degree of polarization of high accuracy.

A plurality of integrator lenses are provided in series to form an integrator lens group. A multiplicity of such integrator lens groups are provided in parallel in the form of compound eyes. Polarizing prisms are arranged for every integrator lens group. The polarizing prism is arranged on the inner side or outer side of each integrator lens. The cementing surfaces of the adjacent polarizing prisms are arranged so as not to be paralleled to each other, and so as to be made, for example, perpendicular to each other.