An object of the present invention or a problem to be solved by the present invention is to provide, as a material for use as a positive electrode of a sodium secondary battery, a novel material that allows the resulting battery to have capacity characteristics superior to those of conventional batteries. The composite metal oxide of the present invention has a composition represented by the general formula NaxMeyO2, where Me is at least one selected from the group consisting of Fe, Mn, and Ni, x satisfies 0.8