There is provided a conductive polymer having a high electrical conductivity and an excellent heat resistance. Using it as a solid electrolyte, there is provided a solid electrolyte capacitor having a low ESR and a large capacitance with good reliability under a hot condition. A monomer mixture of 2,3-dihydro-thieno[3,4-b][1,4]dioxin and 2-alkyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin at a mixture ratio of 0.05:1 to 1:0.1 by the molar ratio is polymerized in the presence of an organic sulfonic acid, and the organic sulfonic acid is included as a dopant. As the 2-alkyl-2,3-dihydro-thieno[3,4-b][1,4]dioxin, the alkyl portion can be methyl, ethyl, propyl or butyl.