PURPOSE:To extend the operating life of optical parts and to reduce an overall cost by operating the optical parts coated with the optical film of arsenic triselenide in an inert gas under the irradiation of a laser. CONSTITUTION:An antireflecting film 2 coated to both end of a fiber 1 is constituted of the arsenic triselenide films 3, 5 and a lead fluoride layer 4 and seals the fiber 1 in an armored cable 6 which is shielded from the outdoor air by a window 7 and a condenser lens 8. The laser beam 12 is introduced through the window 7 and the film 2 into the fiber 1 and is emitted from the lens 8 side. The insert gas such as dry gaseous nitrogen is thus sealed into the cable 6 and the antireflecting film of the arsenic triselenide is formed and therefore the operating life of the optical film is considerably extended.