To improve a projection optical system for a projection exposure system so that a constituent element of at least one refracting sub unit of the projection optical system can be efficiently utilized by a simple design of the projection optical system.

The projection optical system 6 including at least one curved mirror M1-M6, maps an object field at an image field in an image plane 8. The projection optical system 6 includes at least one refracting sub unit 27 in an image forming beam path between an object plane 4 and the image plane 8. A reflecting surface of at least one mirror M1-M6 of the projection optical system 6 is constituted as a static free curved surface undescribable by a rotational symmetric function.