In an illumination optical system arranged to illuminate an illumination target plane by using light from a light source, the illumination optical system includes a prism unit arranged to refract the light, an optical integrator arranged to form a plurality of light sources with light emerging from the prism unit, and an optical system arranged to introduce light emerging from the optical integrator to the illumination target plane. The prism unit includes a pair of conical refractive surface having a conical concave refractive surface and a conical convex refractive surface, and a pair of pyramidal refractive surface having a pyramidal concave refractive surface and a pyramidal convex refractive surface. At least one of the concave refractive surface and the convex refractive surface of the one pair is disposed between the concave refractive surface of the other pair and the convex refractive surface of the other pair.