PURPOSE: To decrease corrosion and blockade of an exhaust gas duct and to utilize NH3 effectively so as to decrease NOx, by injecting NH3 less than a specific quantity which is less than the mols of NOx contained in the exhaust gas.

CONSTITUTION: NH3 less than 0.4mol per 1mol NOx contained in exhaust gas is injected into an exhaust gas duct nearly constantly and radially through a nozzle located at the substantial center of the exhaust gas duct. The quantity of NH3 is sufficient to remove NOx. The duct or air-heater is never corroded or blocked with the ammonium sulfate or ammonium hydrogensulfate deposited on them because little NH3 remains unreacted at a temp. lower than the m.p. of the cpds., hence these cpds. are hardly formed. In addn., no special equipment for treating the remaining NH3 is required.