There is provided a production method of a lithium-containing composite oxide capable of improving performances of cycle characteristics, rate characteristics, and the like of a lithium ion secondary battery. A production method of a lithium-containing composite oxide is characterized in that when producing a lithium-containing composite oxide by mixing a transition metal hydroxide containing Ni and Mn essentially and a lithium source and heating the mixture, a transition metal hydroxide having a crystallite diameter of the (100) plane being 35 nm or less in a crystal structure model in the space group P-3m1 of an X-ray diffraction pattern is used.