PROBLEM TO BE SOLVED: To provide a method for forming an anodically oxidized film excellent in insulation performance and corrosion resistance, on aluminum or an aluminum alloy constituting a CVD apparatus which is used in a high temperature atmosphere and an etching apparatus which uses a strong corrosive gas.SOLUTION: There is provided a method for forming an oxidized film on a surface of a base material composed of aluminum or an aluminum alloy by immersing the base material in an alkali solution and performing an anodic oxidation treatment accompanied by spark discharge. The method comprises a step A of performing the anodic oxidation treatment at a constant current density until a voltage becomes a first voltage which is 200 V or higher, a step B of performing the anodic oxidation treatment while lowering the voltage linearly or in a stepwise manner in a predetermined time from the first voltage to a second voltage which is a voltage lower than the first voltage without performing the constant voltage tr
