Relating to a 5 V-class spinel type lithium nickel manganese-containing composite oxide having an operating potential of 4.5 V or more with respect to a Li metal reference potential, the present invention proposes a composite oxide being capable of improving cycle properties while suppressing the amount of gas generation under high temperature environments and of increasing thermodynamical stability of a positive electrode in a fully charged state. Proposed is a spinel type lithium nickel manganese-containing composite oxide represented by a general formula [Li(LiaNiyMn2-a-b-y-z-αTibAlzMα)O4-σ] (where 0