There is provided an oxidant dopant agent for conductive polymer production, and a solution thereof, in which they are capable of producing a conductive polymer suitable in producing an electrolyte capacitor having a low leak current. There is also provided a conductive polymer prepared by using either of them. There is also provided an electrolyte capacitor having a low leak current. The oxidant dopant agent includes: an organic ferric sulfonate; and at least one kind selected from the group consisting of phosphoric acid, phosphate, phosphorous acid, phosphite, boric acid, borate, thiophosphoric acid, thiophosphate, dithiophosphoric acid and dithiophosphate. The solution includes the oxidant dopant agent dissolved in water, an alcohol or a mixture of water and an alcohol. A conductive polymer is prepared by means of oxidation polymerization of a monomer such as thiophene or its derivative. The conductive polymer is used as an electrolyte to provide an electrolyte capacitor.