Provided is a new 5 V-class spinel-type lithium-manganese-containing composite oxide capable of achieving both the expansion of a high potential capacity region and the suppression of gas generation. Proposed is the spinel-type lithium-manganese-containing composite oxide comprising Li, Mn, O and two or more other elements, and having an operating potential of 4.5 V or more at a metal Li reference potential, wherein a peak is present in a range of 14.0 to 16.5° at 2θ, in an X-ray diffraction pattern measured by a powder X-ray diffractometer (XRD) using CuKα1 ray.