The present invention provides an agent serving as oxidant and dopant in which it does not generate precipitates in a state of an alcohol solution for a certain period of time and the oxidizing power is not too strong. By using the same, the present invention provides a conductive polymer having a high conductivity and heat resistance. By using the same, the present invention provides a solid electrolytic capacitor reliable under a high temperature condition. The present invention provides an agent serving as oxidant and dopant for conductive polymer production, the agent serving as oxidant and dopant comprising: a ferric salt of acids comprising a benzene sulfonic acid derivative represented by formula (1) and sulfuric acid. “R” represents an alkyl or alkoxy group having a carbon number of 1 to 4. More than 90 mole % of the benzene sulfonic acid derivative have R at the para position with respect to the SO3H group. Sulfuric acid is contained at 0.05 to 1.5 mass % in the acids. An alkali metal and alkali earth metal as an inorganic alkali component are included at a total content of 300 ppm or less (based on mass). ammonia and an amine as an organic alkali component are included at a total content of 0.01 mole % or less. A pH value is 1.5 to 3.0 when the agent serving as oxidant and dopant is made into an aqueous solution at a concentration of 8 mass %.