An object of the present invention is to provide a positive electrode active substance for a lithium secondary battery, the positive electrode active substance, when being used as a positive electrode active substance for a lithium secondary battery, being little in deterioration of cycle characteristics and being high in the energy density retention rate, even in repetition of charge and discharge at high voltages, and a lithium secondary battery little in deterioration of cycle characteristics and high in the energy density retention rate, even in repetition of charge and discharge at high voltages. The positive electrode active substance for a lithium secondary battery comprises a lithium cobalt-based composite oxide particle having a Ti-containing compound and an Mg-containing compound adhered on at least part of the particle surface.