A laser beam emitted from an exposure laser beam source has a uniform illuminance distribution after passing through a fly-eye lens selected according to the aperture stop. The exposure light is projected onto a reticle on which a predetermined pattern is formed, and the reticle pattern is projected onto a wafer by means of a projection optical system having a pupil filter inserted therein. A cleaning light beam branching off from the exposure light beam is projected through a beam splitter and a total reflection mirror onto the fly-eye lens located at a retraction position. A plane-parallel plate at the retraction position is irradiated with a cleaning light beam. The fly-eye lens and the plane-parallel plate are self-cleansed by the cleaning light beam, so that the transmissivity is kept higher than a predetermined value during standby.