Disclosed is a polymerization solution for electrolytic polymerization having a small environmental load, having excellent economic efficiency and capable of producing a conductive polymer exhibiting excellent heat resistance. The polymerization solution has: a solvent consisting of 100 to 80% by mass of water and 0 to 20% by mass of an organic solvent; at least one monomer selected from the group consisting of 3,4-disubstituted thiophenes; and at least one organic non-sulfonate supporting electrolyte having an anion with the molecular weight of 200 or more. A conductive polymer film densely filled with polymer particles is obtained by performing electrolytic polymerization using the polymerization solution. A polymer electrode provided with the conductive polymer film exerts excellent heat resistance and the electrochemical activity of the polymer electrode will hardly deteriorate even when being subjected to high temperatures. Moreover, a solid electrolyte capacitor containing the conductive polymer layer obtained by performing electrolytic polymerization using the polymerization solution exhibits low dielectric loss and equivalent series resistance and exerts excellent heat resistance.