The corrosion of an aluminum (Al) layer vacuum-deposited onto the electropolymerized poly(3-methylthiophene) (PMeT) film grown on the gold electrode was studied by visible absorption, ESR spectroscopy and XPS and by SEM analysis from the viewpoint of the redox reaction in the dark at the two-layer interface between Al and PMeT for the Al/doped PMeT/Au sandwich cell. The electrochemical undoping of the perchlorate anions from a PMeT induced a remarkable suppression of the redox reaction at the two-layer interface leading to avoid a corrosion of Al electrode vacuum-deposited onto the PMeT film. It was suggested that the rectifying and photovoltaic effects for the Au/undoped PMeT/Al sandwich cell might be explained under the light of the redox reaction between Al and PMeT under biased voltage and under photo-illumination, respectively.