PURPOSE: To prevent accumulation of scales due to continuous cooling in a specified area of a wall to which a reducing agent soln. touches.

CONSTITUTION: In a denitrification device, an ammonium aq. soln. or nitrogen- contg. org. compd. as a reducing material for nitrogen oxides is atomized from a line 7 into a flue 2 of combustion exhaust gas to make fume. This fume is vaporized and made to react with nitrogen oxides in the exhaust gas to convert the gas into harmless nitrogen. The atomizer for the reducing material is equipped with a device to add the reducing material for denitrification of exhaust gas. This device can periodically change the distribution of the fume in the exhaust gas flow while maintaining the amt. of the reducing material added to the exhaust gas to a desired value. In figure 3, as an example, two nozzles of nozzle 31 in the upstream side and a nozzle 32 in the downstream side are controlled to be alternately used with a timer 23. Temp. in the downstream area near each nozzle 31, 32, is repeatedly raised and decreased, so that these areas are not maintained at low temp.