PROBLEM TO BE SOLVED: To provide a composite metal oxide for forming a nitrogen oxide decomposition catalyst having sufficiently high decomposition activity on nitrogen oxides.SOLUTION: The composite metal oxide contains Ce (cerium) and Nb (niobium) and the composite metal oxide essentially comprises a cerium niobium double oxide represented by compositional formula of CeNbOx, where x represents a number of 4.03 or more and 4.30 or less. In an X-ray diffraction pattern using CuKα line obtained by X-ray diffraction measurement of the composite metal oxide, a ratio (A/B) is 0.5 or more, which is a ratio of the total peak area (A) of diffraction peaks of the cerium niobium double oxide present in a region where 2&thetas; is 25.0 to 30.0° to the total peak area (B) of diffraction peaks present in the above region; and the composite metal oxide has a BET specific surface area of 10 m/g or more.SELECTED DRAWING: None
