An exposure apparatus for exposing a substrate to a pattern of a mask by use of exposure light. The apparatus includes an optical system for directing the exposure light from a light source to the substrate, the optical system having an optical element, a first casing for accommodating therein an optical surface of the optical element, and a second casing for accommodating therein the optical element and the first casing, a first port provided in the first casing, a second port provided in the second casing, a supplier for supplying an inert gas into the first casing and the second casing, and a supplier for supplying an inert gas into the first casing and the second casing through the first port and the second port.