Provided is an anode oxide processing method in which the generation of cracks is suppressed in an anode oxide film formed on an aluminum alloy substrate surface, such as an inner wall of a vacuum chamber of a plasma processing device, and an anode oxide film having low heat reflectivity and a high withstand voltage is formed with high efficiency. The method for forming an anode oxide film involves forming the anode oxide film on the surface of a JIS 6061 aluminum alloy substrate in a sulfuric acid solution or a mixed acid solution of sulfuric acid and oxalic acid. The total voltage in the direction of the film thickness is at least 1650 V[mu]m for the entire film thickness of the anode oxide film formed. In the method for forming an anode oxide film in which the anode oxide film from the boundary surface of the aluminum alloy substrate with the anode oxide film to the surface of the anode oxide film and the 25 [mu]m position in the film thickness direction is formed at no more than the electrolysis voltage of 27 V, and the total voltage from the boundary surface to the 25 [mu]m position in the film thickness direction is at least 820 V[mu]m and no more than 1000 V[mu]m, an anode oxide film having a high withstand voltage can be formed to satisfy the heat reflectivity, crack density, processing time, and the desired standards.