Provided are: a composite metal oxide with which it is possible to improve the performances of a sodium secondary battery; a process for producing the composite metal oxide; a positive active material which comprises the composite metal oxide; a positive electrode produced using the positive active material; and a sodium secondary battery equipped with the positive electrode. The composite metal oxide is represented by formula (I) and is configured of an oxide having a P2 structure and a lamellar oxide having an octahedral structure and/or a triangular-prism structure, which each functions as a stacking fault. The lamellar oxide preferably is an oxide having an 03 structure.