To obtain a magnesium-fluoride optical thin film which has superior adhesive strength and is useable even at a place where a load is placed by preventing the film from peeling against a specific load in a sticking strength test using a scratch tester and forming it by a nonheating method.

The film is prevented from peeling against a load of 40 gf in a sticking strength test using a scratch tester and formed by a nonheating method. To improve temporal stability of optical characteristics of this optical thin film, the Is density in an inorganic compound is preferably 3 to 10 wt.%. This optical thin film has a low refractive index. This optical thin film can suppresses variation in molecular weight due to oxygen ions during sputtering since molecular refraction is varied (made small in coupling angle) as a method for lowering the refracting index, and the refractive index does not become large unlike a film by an IAD method (increases when the molecular volume becomes large as oxygen ions re taken in), thereby obtaining a refractive index close to that of an MgF2 vapor-deposited film.