An optical integrator of a wavefront dividing type permits an arbitrary distance to be set between an entrance surface and an exit surface, without production of aberration and without reduction in reflectance on reflecting films. The optical integrator has a plurality of first focusing elements (first concave reflector elements 18a) arranged in parallel, a plurality of second focusing elements (second concave reflector elements 20a) arranged in parallel so as to correspond to the first focusing elements, and a relay optical system (19) disposed in an optical path between the first focusing elements and the second focusing elements. The relay optical system refocuses a light beam focused via one of the first focusing elements, on or near a corresponding second focusing element so as to establish an imaging relation of one-to-one correspondence between one of the first focusing elements and one of the second focusing elements.