A projection optical system is provided for projecting a pattern on an object surface onto an image surface in a reduced size. The projection optical system has six reflective surfaces that include, in order of reflecting light from the object surface, a first reflective surface, a second convex reflective surface, a third convex reflective surface, a fourth reflective surface, a fifth reflective surface and a sixth reflective surface. An aperture stop is provided along an optical path between the first and second reflective surfaces. The following condition is met by the system, where L1 is an interval between the object surface and the surface apex that is the closest to the object surface, and L2 is an interval between the surface apex of the first reflective surface and the surface apex that is the closest to the object surface:<< ##EQU00001##