A catadioptric projection optical system for forming a reduced image of a first surface (R) on a second surface (W) is a relatively compact projection optical system having excellent imaging performance as well corrected for various aberrations, such as chromatic aberration and curvature of field, and being capable of securing a large effective image-side numerical aperture while suitably suppressing reflection loss on optical surfaces. The projection optical system comprises at least two reflecting mirrors (CM1, CM2), and a boundary lens (Lb) whose surface on the first surface side has a positive refracting power, and an optical path between the boundary lens and the second surface is filled with a medium (Lm) having a refractive index larger than 1.1. Every transmitting member and every reflecting member with a refracting power forming the projection optical system are arranged along a single optical axis (AX) and the projection optical system has an effective imaging area of a predetermined shape not including the optical axis.