PROBLEM TO BE SOLVED: To provide a formation method of an iron oxide (triiron tetraoxide) film, which maintains hardness of a substrate (a ferrous material or a ferrous component), suppresses formation of iron sesquioxide as much as possible, and can effectively form a triiron tetraoxide film in relatively short time, and the iron oxide film using the method.SOLUTION: In a formation method of an iron oxide film according to the invention, a substrate 5 (a ferrous material or a ferrous component) is placed in a film deposition chamber 3 within a vacuum film deposition apparatus 1 including the film deposition chamber 3 and a pressure gradient type electron gun 4. Then an iron oxide (FeO)film is formed on a surface of the substrate 5 by introducing oxygen gas to the film deposition chamber 3 while a plasma region 20 is generated in the film deposition chamber 3 using the pressure gradient type electron gun 4 by applying a negative bias voltage to the substrate 5 (a ferrous material or a ferrous component).
