To provide an illumination optical system capable of almost uniformly adjusting the pupil intensity distribution on respective points of a surface-to-be-illuminated.

An optical integrator (9) arranged in an optical path of an illumination optical system for illuminating surfaces-to-be-illuminated (M; W) by light from a light source (1) comprises a plurality of first refraction surfaces having a predetermined refractive power in a Z direction and a plurality of second refraction surfaces formed behind them and having a predetermined refractive power in the Z direction. At least between two adjacent second refraction surfaces, a dimming section is arranged which has a dimming rate property with a dimming rate increased as a position in the surface-to-be-illuminated to which the light reaches is away from the center of the surface-to-be-illuminated along the Y direction.