To solve the problem of difficulty in the formation of a polymerized film having a sufficient thickness in the conventional method for forming an insulating polymer thin film by using an electrolytic polymerization because the surface of a conductor is insulated by the growth of the film.

The metal conductor is dipped in an aqueous solution containing a polymerizable monomer, a supporting electrolyte and an alkylamine, and an electroconductive polymer film is formed by the electrolysis by using the metal conductor as an electrode. The dopant is eliminated from the obtained electroconductive polymer film to provide the objective polymerized film excellent in insulation properties on the electrode surface.