Provided is a composite metal oxide which is represented by Formula (1) and has an α-NaFeO2 type crystal structure, in which a peak half value width of a (104) plane to be measured by powder X-ray diffraction is 0.250° or less at 2θ. NaxM1r(FeyNizMnwM1−y−z−w)O2±δ  (1) (in Formula (1), M represents any one or more elements selected from the group consisting of B, Si, V, Ti, Co, Mo, Pd, Re, Pb, and Bi, M1 represents any one or more elements selected from the group consisting of Mg and Ca, and relations 0≤r≤0.1, 0.5≤x≤1.0, 0.1≤y≤0.5, 0