A surface of a substrate is coated with a sol solution containing metal alkoxide. A gelled material of the sol solution is pressed against a molding tool while the gelled material is kept soft. Thus, the gelled material is molded. Then, the gelled material is hardened. As a result, protrusions of the hardened gel layer 14 are formed on the substrate so that an irregular texture substrate is formed. A multilayer thin film constituted by a plurality of layers of at least two kinds of materials with different refractive indexes is laminated on a surface of the irregular texture substrate.