An object of the present invention is to provide a novel sulfur-based positive-electrode active material which can largely improve cyclability of a lithium-ion secondary battery, 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: a carbon skeleton derived from a polymer composed of a monomer unit having at least one hetero atom-containing moiety, and sulfur incorporated into the carbon skeleton as the carbon skeleton is formed from the polymer by heat treatment; wherein the hetero atom-containing moiety is pyrrolidone or the polymer is polyvinylpyridine.