A lithium secondary battery using lithium manganese oxide as a positive active material and having excellent charge and discharge cycle properties. As a positive active material of a lithium secondary battery, lithium manganese oxide having a cubic spinel structure, in which the strength ratio (P₂/P₁ strength ratio) of the primary endothermal peak (P₁) appearing around 950 C. and the secondary endothermal peak (P₂) appearing around 11000 C. in differential thermal analysis is under 1, is used.