Disclosed herein is a positive electrode active material prepared by mixing a lithium-containing compound, a compound containing a transition metal to be put into a solid solution, and a compound containing a metallic element M2 different from the transition metal, and firing the mixture to form composite oxide particles, depositing a compound containing at least one element selected from among sulfur (S), phosphorus (P) and fluorine (F) on surfaces of the particles, and firing the particles, whereby each of the particles is provided with a concentration gradient such that the concentration of the metallic element M2 increases from the center toward the surface of the particle, and at least one element selected from among (S), (P) and (F) is made present in the form of being aggregated at the surfaces of the composite oxide particles.