PROBLEM TO BE SOLVED: To prevent the diffusion of water and oxygen from a dielectric protective film by eliminating free oxygen from a thin oxide film without detriment to the characteristics of the film as a dielectric protective film. SOLUTION: A thin film of a mixed oxide comprising niobium oxide and silicon oxide or comprising niobium oxide and titanium oxide is used as a dielectric material for forming a dielectric protective film for an optical disk etc. In a desirable example, a target comprising niobium oxide as a main component and 1 to 30 wt.% of silicon oxide is used in the formation of an oxide thin film by sputtering. In this case, the oxide thin film is formed desirably in a nitrogen atmosphere. A nitrogen-containing oxide thin film is made by sputtering an oxygen-deficient target in the presence of a small amount of added nitrogen. By this way, it is possible to make a thin film having low reducibility and consequent high barrier properties as well as properties equivalent to those of a perfect oxide. COPYRIGHT: (C)2006,JPO&NCIPI