To provide an illumination optical apparatus capable of adjusting the quantity of illumination light while maintaining a constant uniformity of illumination at a light irradiation face, a constant numerical aperture of incident light flux, and a constant color temperature.

The illumination optical apparatus is provided with a light source 11; an optical system arranged between the light source 11 and the light irradiation face 10a and including a collector lens 12, an afocal system 13, a fly-eye lens 14, and a condenser lens 15 sequentially from the side of the light source for telecentrically illuminating the light irradiation face; and a diameter-variable diaphragm member 16 arranged in the afocal system.