A lithium secondary battery that is highly safe and has long life. The battery, which is a nonaqueous lithium secondary battery, utilizes a cathode active material comprising a complex oxide material having a layered structure containing at least Li and Ni and being represented by the chemical formula Li.sub.xNi.sub.a(Mn.sub.yM.sub.1-y).sub.b(CO.sub.zM'.sub.1-z)cO.sub.2(0&l- t;x<1.2, 0