PURPOSE: To obtain an optical thin film of high quality by using a mixture of niobium pentoxide with zirconium oxide so as to prevent the scattering of niobium pentoxide during vapor depostion.

CONSTITUTION: An optical thin film is composed of niobium pentoxide and zirconium oxide. When the molar ratio of niobium pentoxide to zirconium oxide is ≥1, the film shows ≥2.0 high refractive index. A mixture of niobium pentoxide powder with zirconium oxide powder is molded under high pressure, and the molded body is hot pressed in a vacuum furnace at about 10-2Torr degree of vacuum to form a sintered pallet. A substrate on which vapor is deposited is kept at a temp. 20°C below room temp. to 80°C, and the pallet is heated as chemicals for vapor deposition with an electron gun at about 10-5Torr degree of vacuum. Said optical thin film can be formed on the substrate without sticking fine granules. The film is effectively used as an antireflection film in the field of optical disk or photomagnetic disk technique.