An object of the present disclosure is to provide a new sulfur-based positive-electrode active material which can improve cyclability of a lithium-ion secondary battery while maintaining a charging and discharging capacity, a positive-electrode comprising the positive-electrode active material, and a lithium-ion secondary battery comprising the positive-electrode. The sulfur-based positive-electrode active material is one comprising doped nitrogen atoms obtainable by heat-treating a starting material comprising a chain organic compound and sulfur under an atmosphere of a nitrogen atom-doping gas.