An exposure apparatus includes a light source, which emits an exposure beam, an optical system including a casing and an optical element, the casing having a closed space and the optical element being disposed in the casing, a first supplier, which supplies an inert gas into the closed space, a second supplier, which supplies one of oxygen and clean air into the closed space and a controller, which changes a wavelength of the exposure beam between exposure of a substrate and cleaning of the optical element.