To provide a composition for forming an aluminum oxide film by which a non-porous oxide film is formed on an alloy of aluminum or the like without using a harmful material for sealing treatment and causing environmental pollution and environmental destruction does not occur because an electrolyte contains no nitrogen-containing material and to provide the composition for forming the aluminum oxide film by which a film for base coating having excellent adhesion to a coating material is applied on aluminum and an aluminum oxide film used for a capacitor is formed.

The aluminum oxide film 3 formed by electrolysis in an aqueous solution 5 containing aluminum ion is provided on aluminum or the alloy. The aqueous solution contains aluminum sulfate as the aluminum ion source and sulfuric acid for pH adjustment. A cathode electrolysis is used to form the aluminum oxide film 3.