A non-aqueous electrolyte secondary battery uses as its positive electrode active material a mixture of a first lithium-containing transition metal oxide containing nickel and manganese as transition metals and having a crystal structure belonging to the space group R3m and a second lithium-containing transition metal oxide containing nickel, cobalt, and manganese as transition metals and having a crystal structure belonging to the space group R3m, or a mixture of the first lithium-containing transition metal oxide and a lithium cobalt oxide. The first lithium-containing transition metal oxide is Li.sub.aNi.sub.xMn.sub.yO.sub.2 wherein 1.ltoreq.a.ltoreq.1.5, 0.5.ltoreq.x+y.ltoreq.1, 0