A positive active material is provided which can give a battery having a high energy density and excellent high-rate discharge performance and inhibited from decreasing in battery performance even in the case of high-temperature charge. Also provided is a non-aqueous electrolyte battery employing the positive active material. The positive active material contains a composite oxide which is constituted of at least lithium (Li), manganese (Mn), nickel (Ni), cobalt (Co), and oxygen (O) and is represented by the following chemical composition formula: Li.sub.aMn.sub.bNi.sub.cCo.sub.dO.sub.e (wherein 0