A nonaqueous electrolyte secondary battery according to the invention comprises a positive electrode containing a positive electrode active material including lithium containing composite oxide having a layer crystal structure represented by a general formula of Li.sub.xMn.sub.aCo.sub.bM.sub.cO.sub.2 (0.9.ltoreq.X.ltoreq.1.1, 0.45.ltoreq.a.ltoreq.0.55, 0.45.ltoreq.b.ltoreq.0.55, 0