PURPOSE: To prevent the generation of a defective short-circuit due to the deviation of winding by forming an aluminum oxide insulating film on the surface of an aluminum case.

CONSTITUTION: When the removal of the press oil of the case for a press- worked aluminum electrolytic capacitor and an insulating film forming treatment are simultaneously conducted, the surface of aluminum is oxidized by dipping the material into the chemical solution mainly composed of sodium silicate. As a result, the press-oil removal and the insulating film formation can be conducted simultaneously. As an aluminum oxide insulating film is formed on the surface of an aluminum case 1, a defective short-circuit can be prevented even when there is a winding deviation on the inside capacitor element 2.