The present invention provides a reflective control switchable thin film material that can become completely colorless when transparent, and the present invention relates to a reflective control switchable thin film material composed of a multilayer thin film in which a magnesium-titanium alloy thin film is used, and to reflective light control glass, and light control window glass, wherein a magnesium-titanium alloy thin film is used as a control switchable layer, a catalyst layer is formed on the surface of the thin film, a protective layer is optionally formed over the catalyst layer, the material has chromic characteristics whereby a colorless and transparent state is produced by hydrogenation at room temperature (about 20.degree. C.), and the material has chromic characteristics whereby a mirror state is produced by dehydrogenation at room temperature (about 20.degree. C.), and thereby a novel magnesium-titanium alloy thin film material that exhibits excellent reflective control switchable characteristics and is colorless when transparent is provided.