PURPOSE: To produce an oxide film having an arbitrary refractive index by sputtering simultaneously Ta2O5 and SiO2 targets while controlling the ratio between electric powers for sputtering applied on the targets.

CONSTITUTION: A target 4 of Ta2O5 having a high refractive index and a target 5 of SiO2 having a low refractive index are set near a substrate holder 2 in a vacuum chamber 1 and the targets 4, 5 are connected to high frequency power sources 6, 7. The holder 2 is rotated, such as Ar is introduced and the targets 4, 5 are simultaneously sputtered. During this sputtering, electric powers applied on the targets 4, 5 with the power sources 6, 7 are controlled and a film is formed on a substrate 3 while mixing the components of the targets 4, 5 in a constant ratio. An oxide film having an arbitrary uniform refractive index in a wide range or an arbitrary refractive index distribution can be obtd.