A lithium ion secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode active material, a conductive agent and a binder, the positive electrode active material comprises a lithium-containing composite oxide represented by the formula Li.sub.a(Co.sub.1-x-yMg.sub.xM.sub.y).sub.bO.sub.c, where M is at least one selected from Ni and Al, 0.ltoreq.a.ltoreq.1.05, 0.03.ltoreq.x.ltoreq.0.15, 0.ltoreq.y.ltoreq.0.25, 0.85.ltoreq.b.ltoreq.1.1 and 1.8.ltoreq.c.ltoreq.2.1, and the amount of the conductive agent contained in the positive electrode is not more than 3.0 parts by weight per 100 parts by weight of the positive electrode active material.