An optical system has a plurality of optical surfaces and an aperture stop, and has an eccentric reflective surface as at least one of the plurality of optical surfaces. For different conjugate distances, focusing for keeping a position of an entire image surface substantially constant while keeping substantially constant an axial imaging position and an angle of incidence of an axial principal ray on an image surface is performed by parallel and rotational movement of a group provided as a focusing group having at least one surface as the eccentric reflective surface.