PURPOSE: To obtain a thin film which obviates a change of optical characteristics in spite of a change in using environment, more particularly a change in humidity and obviates the change of the optical characteristics in spite of using high-power light by eliminating the moisture adsorbed on the thin optical film and protecting the film with a hermetic housing.

CONSTITUTION: The hermetic housing consists of two sheets of substrates (made of glass or quartz) disposed to face each other and a sealing material sealing the outside circumference thereof. Sealing glass having a low m.p. or synthetic resin sealing material is sued for the sealing material. The inside of the hermetic housing may be a vacuum or under the reduced pressure or a pressurized dry gas, for example, air or inert gas such as nitrogen may be packed therein. The elimination of the moisture to the tin optical film is executed by manufacturing said film in a manner as to avoid the adsorption of the moisture thereto or heating the film at a high temp. or resting the same for the prescribed time or above in a vacuum or exposing the film in the flow of the dry gas after the manufacture. The resulted thin optical film which is not adsorbed with th moisture is disposed in the hermetic housing in which working environment (in the vacuum, dry gas or high temp. gas) where the moisture is not adsorbed again on the film.