A projection optical system is a catoptric system in which a field of view region and an imaging region are located spaced from an optical axis, in which a numerical aperture of light reaching each point on an image plane is substantially uniform regardless of an image height and a direction. An aperture stop for defining the numerical aperture of the projection optical system is provided, and the aperture stop is provided with an aperture portion in a predetermined shape in which the numerical aperture of light reaching each point within a predetermined region is substantially uniform over the predetermined region, that is, in a shape in which dimensions concerning two directions perpendicular to each other are different from each other. A predetermined shape of the aperture portion is defined so as to compensate for the effect of non-uniformity of the numerical aperture of light reaching each point within a predetermined region due to a partial optical system arranged between the aperture stop and an image plane not satisfying a desired projective relationship.