To provide a method for forming a protective film having high corrosion resistance and suitable for the inner wall members of a CVD system, an etching system or the like by making an anode oxide film of aluminum dense.

In this method, warm water 46 of 70C to 90C (a first temperature) is brought into contact with an anode oxide film 12 to form a discontinuous face on the surface of the anode oxide film 12, and then hot water of a second temperature, which is above the first temperature, or a water vapor having the second temperature into contact with the film. The hot water or the water vapor is introduced through the surface of the anode oxide film 12 into the inside of the anode oxide film 12. Accordingly, voids on the surface portion in the anode oxide film 12 are clogged by a hydrate formed upon the contact of the voids with the hot water or the water vapor. Therefore, a protective film formed of a dense layer can be reliably formed on the surface of the anode oxide film 12.