To provide an optical system machining device and a machining method using it capable of keeping the surface of an optical system member in an initial cleaning state by eliminating the deterioration of the cleaning state owing to contamination and capable of improving irradiation efficiency to a workpiece in the case of machining by photoirradiation by controlling the generation of a heat generation loss and its energy loss owing to contamination of the optical system member.

A nitrogen gas is blown against the outlet part of light in a cover j1 which covers the optical system consisting of a lens g1 or the like so that, for example, only the nitrogen gas supplied as shown by the arrow Y11 comes into contact with the outlet part of light. Whereby, the generation of turbulence owing to the supply of the nitrogen gas is controlled and entangling of the ambient air caused by turbulence is eliminated. Thus, contamination on the surface of the optical system member such as the lens g1 is prevented.