A method of exposure and an aligner wherein yield and productivity in the production of microdevices such as semiconductor devices and liquid crystal display devices can be enhanced by the use of a method of cleaning optical devices, the method capable of satisfactorily cleaning optical devices with high efficiency. This exposure method comprises a step of cleaning optical devices as constituents of illumination optical system and/or projection optical system accommodated in a housing. In the cleaning step, the housing is charged with a gas of moisture content higher than that of a gas charged into the housing at the exposure of photosensitive substrate, and the illumination optical system and/or projection optical system is irradiated with exposure beams (e.g., ultraviolet radiation). In the housing charged with the gas of higher moisture content, water (moisture) present in the housing is exposed to ultraviolet radiation to thereby produce OH radicals exhibiting strong oxidative activity. Organic matter adhering to the surface of optical devices lying in the housing is oxidized and decomposed by the oxidative activity of OH radicals. Thus, cleaning of the optical devices can be accomplished.