The present invention is to provide a cathode active material used for a lithium ion secondary battery which has a large charge-discharge capacity, and excels in charge-discharge cycle properties, output properties and productivity, and, a lithium ion secondary battery using the same. The cathode active material used for a lithium ion secondary battery comprises a lithium-transition metal composite oxide having an α-NaFeO2 type crystal structure and represented by the following formula (1); Li1+aNibCocMdO2+α, where, in the formula (1), M is at least one metal element other than Li, Ni and Co; and a, b, c, d and a are respectively numbers satisfying −0.04≤a≤0.04, 0.80≤b≤1.0, 0≤c≤0.06, b+c+d=1, and −0.2