PURPOSE: To restrain the deterioration such as leakage current generated in soldering process etc., at high temperature without damaging other characteristics by a method wherein a condenser element formed of a film forming metal comprising an anode oxide film is re-synthesized by a synthesizing solution using a non-aqueous solvent.

CONSTITUTION: An anode foil 1 is composed of a film forming metal such as aluminum, etc., formed of a dielectric oxide film by etching-process and synthesis; a cathode foil 2 is also composed of the film forming metal such as etching-processed aluminum; the anode foil 1 of a capacitor element 6 composed of said anode foil 1 and cathode foil 2 holding a paper separator 3 to be wound up is re-synthesized by a synthesizing solution using polyhydric alcohol such as ethylene glycol or diethylene glycol as a solvent and after impregnating a fusion-liquefied organic semiconductor 8 with the synthesizing solution, further cooled down to be set. For example, the capacitor element 6 re-synthesized by the synthesizing solution comprising ethylene glycol with adipic acid ammonium dissolved therein is impregnated with a solution of TCNQ complex salt 8 in an aluminum case 7 which is sealed with a resin 9 and further covered with an epoxy resin 10 etc.