Cathode active material for lithium ion secondary battery

A lithium ion secondary battery having high energy density and of excellent safety, and a cathode active material used therefor are provided. The cathode active material is a Li-containing composite oxide comprising a plurality of transition metal elements selected from Cr, Mn, Fe, Co, Ni and Cu, in which the composition of the transition metal elements is in a range not inclined to particular transition metal elements. The composite oxide having a crystal structure in which the range of an angle ² formed between a axis and b axis of the crystallographic structure is controlled as: 90° < ² ‰¦ 110°. The composite oxide is used as the cathode active material of a lithium ion secondary battery.