An illumination optical system includes a pair of fly-eye mirrors configured to receive light from a light source, a first condenser configured to condense light from the pair of fly-eye mirrors, a reflection type integrator configured to receive light from the first condenser, the reflection type integrator including a plurality of cylindrical reflective surfaces having parallel generating line directions, an aperture stop arranged perpendicular to the generating line direction, and a second condenser configured to superpose on an illuminated surface luminous fluxes from a plurality of cylindrical reflective surfaces of the reflection type integrator.