An illumination optical system which illuminates an illumination surface with light from a light source, includes a divider which divides light from the light source to generate a plurality of light beams, a first reflective integrator which uniformizes light intensity distributions of the plurality of light beams generated by the divider, a condenser which condenses the light beam from the first reflective integrator, a second reflective integrator which receives the light beam from the condenser and illuminates the illumination surface, and an aperture stop arranged between the second reflective integrator and the illumination surface, wherein the divider generates the plurality of light beams so that light beams each having a cross-sectional shape different from a cross-sectional shape of the light provided from the light source to the divider enter a plane on which the aperture stop is arranged.