A conductive polymer having high conductivity and a solid electrolytic capacitor having low ESR are provided. A conductive polymer is synthesized using as a monomer a compound of a dimer to a decamer in which 3-alkyl five-membered heterocyclic rings are bonded at positions 2 and 5, being sterically controlled. Further, this conductive polymer is used as the solid electrolyte of a solid electrolytic capacitor.