An optical system has fluoride compounds provided in an environment exposed by vacuum ultraviolet light or plasma light, which has higher photon energy than an absorption wavelength of a base stock of the optical system. 1-layer of a protective film of SiO2 or metal oxides having a film thickness of 2-20 nm is formed at least on the light irradiation side (inner side) of the optical system to prevent the stripping of the fluorine atoms from the surface of the optical system. In addition, the protective film is a 1-layer film selected from one of SiO2, MgO, TiO2, or ZrO2.