Disclosed is a mixed metal oxide which is reduced in the amount of scarce metal used therein, while having excellent performance as a positive electrode active material for secondary batteries. Also disclosed are a positive electrode for sodium secondary batteries having excellent performance, and a sodium secondary battery. Specifically disclosed is a method for producing a sodium-manganese complex metal oxide which is characterized by comprising a step for calcining a material, which contains sodium carbonate (Na2CO3) and dimanganese trioxide (Mn2O3) in such amounts that the molar ratio of sodium to manganese (Na/Mn) is not less than 0.4 but not more than 0.7, at a temperature of not less than 850° C. Also specifically disclosed are a sodium-manganese complex metal oxide produced by the method, a positive electrode for sodium secondary batteries which contains such a sodium-manganese complex metal oxide, and a sodium secondary battery.