To provide a lithium-containing composite oxide, a cathode active material and a positive electrode for a lithium ion secondary battery, with which a lithium ion secondary battery having favorable cycle characteristics even when charged at a high voltage can be obtained; and a lithium ion secondary battery having favorable cycle characteristics even when charged at a high voltage. A lithium-containing composite oxide which is represented by LiaNibCocMndMeO2 wherein M is Mg, Ca, Al, Ti, V, Nb, Mo, W or Zr, a+b+c+d+e=2, “a” is from 1.01 to 1.10, b is from 0.30 to 0.95, c is from 0 to 0.35, d is from 0 to 0.35, and e is from 0 to 0.05, wherein in an X-ray diffraction pattern obtained by reflection X-ray diffraction employing Cu-Kα rays, the ratio (I104/I110) of the integrated intensity (I104) of a peak of (104) plane to the integrated intensity (I110) of a peak of (110) plane is at least 4.20.