We synthesized 3-dithiolthione-substituted polythiophene, poly [3-(1',2-dithiol-3'-thione-4'-yl)thiophene], by cyclization reaction of poly [3-bis(methoxycaronyl)methyl thiophene]. Its redox behavior and electronic states showed that the oxidation of the polythiophene followed the oxidation of the dithiolthione ring to the radical cation of the dithiolthione ring.