PURPOSE: To obtain an excimer laser device which has reflection protective films of a long life and can be put to a practical use by a method wherein the single layer or multilayer reflection protective films all whose layers are made of fluoride are provided on both the surfaces of a window through which an oscillated laser beam is transmitted out of a housing.

CONSTITUTION: A window for taking out a laser beam is provided in a housing and the window has a reflection of 3.6% (fluorite)∼4.1% (quartz). In order to avoid the reflection, reflection protective films are provided on both the surfaces of the window. The reflection protective film is made of fluoride. As the fluoride, one or more among neodymium fluoride (refractive index n=1.67), magnesium fluoride (n=1.43), thorium fluoride (n= 1.59), strontium fluoride (n=1.46), lanthanum fluoride (n= 1.59) and so forth are selected in accordance with optical design. In order to manufacture the window, for instance, NdF3 thin films (nd=1.66) with optical film thicknesses of λ/4 are formed by vacuum evaporation on both the front and rear surfaces of the window material composed of a fluorite substrate (nd=1.47) as high refractive index thin films and, successively, MgF2 thin films (nd=1.405) with optical film thicknesses of λ/4 are formed by vacuum evaporation as low refractive index thin films to build up double-layer reflection protective films (designed reference wavelength λ=248nm).