A Pt/Pd/CZO catalyst was produced by using 1.78 g of a nitric acid solution of dinitrodiammineplatinum (Pt concentration 4.54 wt %) and 0.43 g of a palladium nitrate solution (Pd concentration 4.4 wt %) for a metal salt solution to be added dropwise to a CZO dispersion used in Comparative Example 4, and in the same steps other than the point as in Comparative Example 4.