A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium-containing composite oxide, wherein the composite oxide is represented by the general formula: Li.sub.zCo.sub.1-x-yMg.sub.xM.sub.yO.sub.2, the element M included in the general formula is at least one selected from the group consisting of Al, Ti, Sr, Mn, Ni and Ca, the values x, y and z included in the general formula satisfy: (i) 0.ltoreq.z.ltoreq.1.03; (ii) 0.005.ltoreq.x.ltoreq.0.1; and (iii) 0.001.ltoreq.y.ltoreq.0.03, the composite oxide has a crystal structure attributed to a hexagonal system in an overcharged state having a potential over 4.25 V relative to metallic Li, and a maximum value of an oxygen generation peak in a gas chromatograph mass spectrometry measurement of the composite oxide in the overcharged state is in the range of 330 to 370.degree. C.