An anode foil and a cathode foil which has an oxidation film layer on the surface are separated by a separator and wound to form a capacitor element and then anodic forming is performed on this capacitor element. Next, the capacitor element is immersed in a solution of less than 10 wt %, preferably between 2.0 and 9 wt %, more preferably between 5 and 8 wt % of polyimide silicone dissolved in a ketone solvent, and after removing, the solvent is evaporated off at between 40 and 100.degree. C. and heat treating is performed at 150 to 200.degree. C. Next, this capacitor element was immersed in a mixture of polymeric monomer and oxidizing agent and the conductive monomer was made to polymerize in the capacitor element to form a solid electrolyte layer. Furthermore, this capacitor element was stored in an external case and the open end was sealed with sealing rubber to form the solid electrolytic capacitor.