A lighting optical system according to the present invention includes a lens optical system (L) or a mirror optical system. The planes (F₁, F₂) of incidence and emergence of the lens optical system (L) and the reflecting surface of the mirror optical system are chosen to satisfy given equations. Thus, it becomes possible to unify illuminance on a surface to be irradiated while sufficiently shortening the optical path length to decrease the device in size, in a close illuminance system, and unify illuminance on an image surface while efficiency utilising the quantity of light without using a gradient filter or the like, in a projective illumination system.