Provided are exceptional covered lithium-nickel composite oxide particles that have lithium ion conductivity, the high environmental stability of said particles making it possible to inhibit the generation of impurities from the absorption of water and carbon dioxide; high adhesiveness also being achieved, and the coating layer not peeling off easily. The covered lithium-nickel composite oxide particles for the positive electrode active material in a lithium ion cell, said particles being obtained by covering the surfaces of nickel-based lithium-nickel composite oxide particles with a polymer composition containing an electron-nonconductive polymer and an electron-conductive polymer, are covered nickel-based lithium-nickel composite oxide particles having excellent environmental stability and no adverse effect on cell characteristics.