NOVELTY - An oxidizing agent-dopant contains ferric salt of acid containing benzenesulfonic acid derivative (1) and sulfuric acid. Content of sulfuric acid is 0.05-1.5 mass%. The oxidizing agent and dopant contains total content of alkali metal and alkaline-earth metal of 300 ppm or less, and total content of ammonia and amines of 0.01 mol% or less. The pH when using aqueous solution having concentration of 8 mass%, is 1.5-3.    USE - Oxidizing agent and dopant is used for synthesis of conductive polymer used as solid electrolyte, battery, antistatic sheet and coating material for solid electrolytic capacitor (both claimed) such as aluminum solid electrolytic capacitor, tantalum solid electrolytic capacitor and niobium solid electrolytic capacitor.    ADVANTAGE - The oxidizing agent and dopant has favorable oxidizing property, and does not produce precipitation for long period of time when prepared in alcohol solution. The alcohol solution of oxidizing agent and dopant has excellent storage stability. The oxidizing agent and dopant provides polymer having high electrical conductivity and excellent heat resistance. Solid electrolytic capacitor comprising the conductive polymer has high reliability even at high temperature.    DETAILED DESCRIPTION - An oxidizing agent-dopant contains ferric salt of acid containing benzenesulfonic acid derivative of formula (1) and sulfuric acid. The content of sulfuric acid is 0.05-1.5 mass% in the whole acid. The oxidizing agent and dopant contains total content of alkali metal and alkaline-earth metal which is an inorganic alkaline component of 300 ppm or less, and total content of ammonia and amines which is an organic alkaline component of 0.01 mol% or less. The pH when using aqueous solution having concentration of 8 mass%, is 1.5-3.    R=1-4C alkyl or alkoxy.    The R group and SO3H group located in para-position of benzene ring is 90 mol% or more with respect to whole benzene sulfonic acid derivative. An INDEPENDENT CLAIM is included for alcohol solution. Oxidizing agent and dopant are dissolved in alcohol having concentration of 30 mass% or more and water content of 5 mass% or less.