There is provided a reflective imaging optical system forming an image of a first plane onto a second plane, wherein the numerical aperture with respect to a first direction on the second plane is greater than 1.1 times a numerical aperture with respect to a second direction crossing the first direction on the second plane. The reflecting imaging optical system has an aperture stop defining the numerical aperture on the side of the second plane, and the aperture stop has an elliptic-shaped opening of which size in a major axis direction is greater than 1.1 times that in a minor axis direction. The reflective image-forming optical system is applicable to an exposure apparatus using, for example, EUV light and capable of increasing numerical aperture while enabling optical path separation of light fluxes.