An X-ray generator introducing an X-ray to an illumination optical system includes plural plasma light sources, and a reflector, movably arranged among the plural light sources, for switching light sources and for reflecting the X-ray from one of the plural light sources to the illumination optical system, wherein an angle between a plane determined by an optical axis of the X-ray emitted from the reflector and a line that connects the plural light sources to the reflector, and a polarization plane on which an electric field vector oscillates is between 45.degree. and 135.degree., the polarization plane maximizing a reflectance to the X-ray of the illumination optical system.