An illumination system includes a first optical integrator of inside reflection type, for reflecting at least a portion of received light, with its inside surface, and for defining a surface light source at or adjacent a light exit surface thereof, a second optical integrator of wavefront division type, for dividing the wavefront of received light and for defining a plurality of light sources at or adjacent a light exit surface thereof, an imaging optical system for imaging the surface light source at or adjacent a light entrance surface of the second optical integrator, and a collecting optical system for superposing light rays from the plurality of light sources one upon another, on a surface to be illuminated, wherein the imaging optical system has a variable imaging magnification.