Abstract: A method of manufacturing an organic semi-conductor solid electrolytic capacitor wherein a conductive polymer thin film layer of a heterocyclic compound, i.e., pyrrole, thiophene or furan, is formed as a solid electrolyte on the surface of a metal substrate having a dielectric oxide film formed thereon by electrolytic oxidation polymerization carried out in an electrolytic solution which contains pyrrole, thiophene or furan as a heterocyclic compound and an aliphatic or aromatic sulfonic acid or a salt thereof as a supporting electrolyte in an organic solvent, for example, acetonitrile, acetone or propylene carbonate.