PURPOSE: To prevent deterioration of polyacetylene by reaction with oxygen, etc., without reducing electric conductivity, by forming a coat on the surface of the polyacetylene by electrolytic oxidative polymerization.

CONSTITUTION: A coat, e.g. polypyrrole film, polythienylene film, etc., is formed on a polyacetylene film by using the polyacetylene film as an anode and a hardly corrodible metal or alloy, e.g. Ni, Pt, etc., or an organic substance having electric conductivity as a cathode in an electrolyte prepared by dissolving at least one supporting base, e.g. Lewis acid and salt of Li, Na, etc., in an organic solvent, e.g. acetonitrile, benzonitrile, etc., in an inert gas atmosphere, e.g. Ar, H2, etc., according to electrolytic oxidative polymerization to improve the oxidative stability of the polyacetylene film.