The conductive polymer of the present invention is prepared by means of oxidation polymerization. On the matrix of the conductive polymer, at least one organic sulfonate formed by an anion of an organic sulfonic acid and a cation of other than transition metals is coated. Alternatively, in the matrix of the conductive polymer, at least one organic sulfonate formed by an anion of an organic sulfonic acid and a cation of other than transition metals is included. The conductive polymer of the present invention is excellent in the conductivity, heat resistance and moisture resistance. By using it as a solid electrolyte, a reliable solid electrolytic capacitor can be prepared which is unlikely to decrease the properties when being kept in a hot and humid condition.