TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Property: The porous silica has specific surface area of 100 m2/g or more.Preferred Composition: The porous silica contains 20-40 wt.% of heteropolyacid with respect to total amount of catalyst. 0.1-10 wt.% of palladium is supported on the porous silica with respect to total amount of catalyst. Methane is supplied to reaction field of nitrogen oxide such that molar ratio of methane and nitrogen oxide is 0.5:1-10:1. In the reaction field of nitrogen oxide, oxygen and 5-20 mol% of water vapor coexist.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Property: The porous silica has specific surface area of 100 m2/g or more.Preferred Composition: The porous silica contains 20-40 wt.% of heteropolyacid with respect to total amount of catalyst. 0.1-10 wt.% of palladium is supported on the porous silica with respect to total amount of catalyst. Methane is supplied to reaction field of nitrogen oxide such that molar ratio of methane and nitrogen oxide is 0.5:1-10:1. In the reaction field of nitrogen oxide, oxygen and 5-20 mol% of water vapor coexist.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The heteropolyacid is H4SiW12O40, H3PMo12O40 or H3PW12O40. EXAMPLE - N-602A (porous silica having specific surface area of 290 m2/g and pore volume of 0.8 cm3/g) was immersed in ethyl alcohol solution of heteropolyacid (H3PW12O40), and the aqueous solution was impregnated into holes of porous silica. The obtained silica was heated, and methyl alcohol was vaporized, to obtain heteropolyacid-containing porous silica (support). The support was immersed in toluene solution of palladium acetate, and the toluene solution was impregnated into holes of silica. The palladium impregnated silica was heated at 120degreesC, toluene was vaporized, and palladium acetate was decomposed. Thus, a reduction catalyst was obtained. The catalyst comprised grains with average particle diameter of 500-800 microns, and contained 30 wt.% of heteropolyacid. 0.2 wt.% of palladium was supported on the support. Nitrogen oxide was efficiently reduced using methane as reducing agent, in presence of the catalyst.