A positive electrode active material for a lithium ion secondary battery is a lithium-containing composite oxide represented by the chemical formula: Li.sub.a(Co.sub.1-x-yMg.sub.xAl.sub.y).sub.bM.sub.zO.sub.c, where M is at least one element selected from the group consisting of Na and K, and the values a, b, c, x, y and z respectively satisfy 0.ltoreq.a.ltoreq.1.05, 0.005.ltoreq.x.ltoreq.0.15, 0.0001.ltoreq.y.ltoreq.0.01, 0.0002.ltoreq.z.ltoreq.0.008, 0.85.ltoreq.b.ltoreq.1.1 and 1.8.ltoreq.c.ltoreq.2.1.This makes it possible to improve a high temperature storage characteristics and safety of the lithium ion secondary battery.