To eliminate the stains which are formed when a bismuthsubstd. rare earth iron-garnet single crystal is cut and to make it possible to produce Faraday rotators for optical isolators at a good yield by constituting the layers in contact with the atm. air of antireflection films consisting of plural thin films, of tantalum pentaoxide.

The layers in contact with the atm. air of the antireflection films consisting of at least the two thin films are composed of tantalum pentaoxide. In such a case, the method for manufacturing the antireflection films is not particularly restricted, and generally practiced vacuum vapor deposition method, ion assisting method, ion plating method, sputtering method, etc., are properly selectable. A low-refractive index film is arranged on a BIG surface and a Ta2O5 is arranged thereon in the case where two layers of the antireflection films are formed. Plural times of the proper repetition of the combinations of the low-refractive index layers and high-refractive index layers with the BIG surface is possible when the conditions of using the Ta2O5 films in the layer in contact with the atm. air are satisfied.