Disclosed is an electrode active material that has a large charge discharge capacity, a high initial efficiency, as well as excellent cycle characteristics and rate characteristics and is favorably used in a non-aqueous electrolyte secondary battery. An organo sulfur-based electrode active material contains sodium and potassium in a total amount of 100 ppm by mass to 1000 ppm by mass; an electrode for use in a secondary battery, the electrode containing the organo sulfur-based electrode active material as an electrode active material; and a non-aqueous electrolyte secondary battery including the electrode. Preferably, the organo sulfur-based electrode active material further contains iron in an amount of 1 ppm by mass to 20 ppm by mass. Preferably, the organo sulfur-based electrode active material is sulfur-modified polyacrylonitrile, and the amount of sulfur in the organo sulfur-based electrode active material is 25 mass % to 60 mass %.