A manganese nickel composite hydroxide which serves as a starting material for positive electrode active materials for secondary batteries, and the secondary battery having low resistance and high output characteristics. A manganese nickel composite hydroxide according to the present invention is represented by general formula (A) Mn1−x−yNixMy(OH)2+α(wherein 0≤×≤0.27, 0≤y≤0.05, 0≤α≤0.5, and M represents at least one element selected from among Mg, Al, Ca, Ba, Sr, Ti, V, Fe, Cr, Co, Cu, Zr, Nb, Mo and W), and has an SO4 content of 0.90% by weight or less, an Na content of 0.04% by weight or less, a BET specific surface area of from 40 m2/g to 70 m2/g (inclusive), and a value obtained by [(d90-d10)/(average particle diameter)] of 0.90 or less, said value being an index indicating the expanse of the particle size distribution.