To provide an illumination optical system capable of excellently illuminating an arcuate area through a small number of mirrors.

The illumination optical system includes a pair of fly-eye mirrors 211 on which light from a light source is incident, a first condenser 212 which condenses light beams from the pair of fly-eye mirrors 211, a corrugated plate integrator 213 which has a plurality of cylindrical reflecting surfaces having aligned generatrix directions and on which light from the first condenser 212 is incident, an aperture diaphragm 215 disposed at right angles to the generatrix directions, and a second condenser 216 which puts light beams from the plurality of cylindrical reflecting surfaces of the corrugated plate integrator 213 one over another on a lit-up surface.