A lithium-containing composite oxide, characterized in that it has a composition represented by the general formula: Li 1+x+alpha Ni(1-x-y+ delta )/2Mn(1-x-y- delta )/2 MyO2, wherein 0 <= x <= 0.05, -0.05 <= x + alpha <= 0.05 and 0 <= y <= 0.4, and 0.1 <= delta <= 0.1 (when 0 <= y <= 0.2) or 0.24 <= delta <= 0.24 (when 0.2 < y <= 0.4), and M is one or more elements selected from the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn; and a nonaqueous secondary cell having a positive electrode using the lithium-containing composite oxide as an active material. The lithium-containing composite oxide has a stable layer crystal structure and a high density, and thus can be used for manufacturing a nonaqueous secondary cell which is excellent in the reversibility of charge and dischargeand in durability and exhibits a high capacity.