PURPOSE: To obtain a thin film having high optical quality by surface-treating an optical device composed of a SiO2-containing material with the generation of a radical reaction and introducing the radical to the surface of the optical device to control reflection factor or transmissivity.

CONSTITUTION: A material to be treated 5 is mounted on a stage 4a in a reaction vessel 1 and after the vessel 1 is evacuated, a reaction gas and an inert gas He are introduced into the vessel 1 to be almost 1-atm. Next, voltage is impressed between the vessel 1 and an electrode 2 to generate He plasma, which is allowed to react with the reaction gas to produce the radical. In this state, the stage 4a is rotated by a moving device 4 to be brought near to the electrode 2, where atomes or moleculars constituting the surface of the material 5 to be treated undergo the radical reaction. In such a time, at least one kind of CeCl4, BCO3 and TiCl4 is contained as the reaction gas in the case the surface refractive index of the material 5 to be treated is increased. And in the case of decreasing the refractive index, at least one kind of SF6, F2, CF4 and NF3 gas is used.