PURPOSE: NOx containing gas is adjusted to a proper temperature under the presence of NH3 gas, and contacted with graphite containing specific metals. The process reduces NOx into harmless N2 with excellent denitrogen ratio, while using hgih strength catalyst resistant to NOx, heat, and oxidation.

CONSTITUTION: NOx containing gas is sucked by the blower 2 and heated in the heat-exchanger or heater 3 to a temperature of 150 to 600°C. NH3 gas is introduced from the inlet 4 in an amount of 2/3 to 2 moles per NOx mole in the waste gas. The mixed gas is introduced into the reactor 5. The catalyst used in the reactor comprises synthetic or natural graphite carrier having a specific surface of 1 to 50M2/g and one or more of catalytic metals selected from the group consisting of V, Fe, Cu, Ti, Co, Ni, Cr, and Mn, with the metals being normally carried in oxide forms. The catalyst amount as metal is 1/100000 or more of graphite amount. The NOx removed, cleaned gas is exhausted from the outlet 6.