The present invention provides a sodium secondary battery capable of reducing the amount of lithium used, and ensuring a larger discharge capacity maintenance rate when having repeated a charge and discharge, as compared with conventional techniques; and a mixed metal oxide usable as the positive electrode active material therefor. A mixed metal oxide of the present invention comprises Na and M1 wherein M1 represents three or more elements selected from the group consisting of Mn, Fe, Co and Ni with an Na:M1 molar ratio being a:1 wherein a is a value falling within the range of more than 0.5 and less than 1. Also, a mixed metal oxide of the present invention is represented by the following formula (1): NaaM1O2  (1) wherein M1 and a each have the same meaning as above. The positive electrode active material for secondary batteries of the present invention comprises the mixed metal oxide above.