PURPOSE: To add Ce with uniform and quantitative controllability and to form a thin film of oxide superconductor having a uniform composition by impregnating a thin rare earth-Cu-O-type film with Ce and applying annealing to the above thin film in a reducing atmosphere.

CONSTITUTION: A thin A-Cu-O-type film (A means lanthanoide series elements or rare earth elements) is formed on a substrate. This thin film is immersed into a Ce-containing solution to undergo impregnation with Ce. Subsequently, annealing is performed in a reducing atmosphere of nitrogen, argon, etc., by which a thin film of A-Ce-Cu-O-type oxide superconductor is formed. By controlling the concentration of the Ce solution and immersion time, the quantitative control of the additive quantity of Ce can be easily carried out. By this method, the superconductivity of the thin film can be improved.