To provide a transition metal-containing hydroxide, which is a precursor of a lithium-containing composite oxide, with which it is possible to obtain a lithium ion secondary battery excellent in the discharge capacity and cycle characteristics, by using as a cathode active material a lithium-containing composite oxide obtained from the hydroxide. A transition metal-containing hydroxide, which is a precursor of a lithium-containing composite oxide, wherein in a distribution of the logarithmic derivative pore specific surface area relative to the pore size, obtained by BJH method, the proportion of the sum of the logarithmic derivative pore specific surface areas with pore sizes of 10 nm or larger, to 100% of the sum of the logarithmic derivative pore specific surface areas in the entire distribution, is at least 23%.