Disclosed is an illumination optical system having an imaging optical system for imaging a predetermined object plane upon an image plane, by use of light supplied from an integrator that functions to produce a plurality of light sources using light from a light source. In one preferred form of the invention, the imaging optical system includes, in an order from a light incidence side thereof, a first lens group having a positive power, a second lens group having a negative power, and a third lens group having a positive power, wherein the second lens group includes a concave lens at a light exit side thereof and wherein the third lens group includes a convex lens at a light entrance side thereof. The position of intersection between an optical axis of the imaging optical system and a principal ray of abaxial light is defined between the second lens group and the third lens group.