To provide an illumination optical system which achieves uniform illuminance distribution on an object surface to be illuminated by combining a light beam from an integrating sphere and a light beam generated by a critical illumination method.

The illumination optical system includes the integrating sphere 1 having, on an optical path from the light source 2 to the object surface 6a, an entrance aperture 11 for introducing a light beam from a light source 2 and an exit aperture 12 for emitting the light which has entered from the entrance aperture 11 and repeated diffuse-reflection inside the sphere to have a uniform luminous emittance. The exit aperture 11 is formed in the integrating sphere 1 so that the optical axis of the illuminating optical system coincides with a line passing through the center of the integrating sphere 1 and the center of the exit aperture 11 and that the light beam from the light source 2 passes through the integrating sphere 1 and illuminates the object surface 6a which is in conjugate relation with the exit aperture 11.