Disclosed is a mixed metal oxide comprising Na, M1, and M2, where M1 represents at least one element selected from the group consisting of Mg, Ca, Sr, and Ba; and M2 represents at least one element selected from the group consisting of Mn, Fe, Co, and Ni, wherein the molar ratio of Na:M1:M2 is a:b:1, where a is a value within the range of not less than 0.5 and less than 1; b is a value within the range of more than 0 and not more than 0.5; and “a+b” is a value within the range of more than 0.5 and not more than 1. An electrode having an active material containing the mixed metal oxide is also disclosed. Further disclosed is an electrode containing the electrode active material as well as a sodium secondary battery comprising the electrode as a positive electrode.