To form a homogeneous thin film for effectively and stably realizing desired characteristics on a base material in simple steps by holding thin film forming substance on a porous material made of normal temperature curable substance.

After water is added to gypsum and agitated, it is cast in a plastic casting mold 1 and dried at ambient temperature. The gypsum 2 in the mold 1 is impregnated with water-repellent treating solution containing perfluoroalkyl group-containing compound. Further, the gypsum 2 is dried, for example, at 70°C, then removed from the mold 1, and installed in a vacuum tank 4 of a vacuum deposition unit. Then, a synthetic resin lens 7 to become base material to be deposited is set in the tank 4. An electron beam 11 is emitted to a sample by an electron beam depositing method by using an electron gun 8 and ZrO2, 9, SiO2 10 provided oppositely, an SiO2 thin film and ZrO2 thin film are alternately laminated to obtain a lens with an antireflection film.