PURPOSE: To produce an aluminium oxide film having a large area by anodizing an aluminium substrate, decreasing then the applied positive voltage stepwise, generating then H2 from the surface of the substrate by increasing the negative voltage successively, then treating again under an anodizing condition.

CONSTITUTION: An aluminium oxide film having a fixed pore diameter is formed under a fixed applied voltage by anodizing in a first stage using an aluminium substrate as anode. In a second stage, the applied anode voltage is decreased a little, and the voltage is further decreased a little after the value of the electrolyzing current is restored almost to a fixed value. The applied positive voltage is brought to ≤3V by repeating this procedure. Then, in a third stage, a negative voltage is applied to the substrate, and the voltage is increased after the electrolyzing current is restored to an almost fixed value. Thus, the substrate is brought to a condition where H2 is generated from the surface of the substrate by repeating the above described procedure. Formed aluminium oxide film is stripped by the generation of H2. When anodization is carried out in a fourth stage by applying a positive voltage again to the substrate, previously formed aluminium oxide film is stripped easily off the substrate.