PURPOSE: To enhance denitration ratio of NOx-containing exhausted high temperature gas without causing second pollution such as NH3, a small quantity of urea solution is injected into the exhaust gas, mixed, cooled, and maintained at cooled condition for a period of specified time.

CONSTITUTION: Exhaust gas is heated to 700 to 1100°C at zones of the prebank 2 or the fire furnace 12 surrounded by walls of water cooling, boilder evaporator pipes 1. Into such high temperature gas zone, aqueous urea solution 13 in a quantity of 0.3 to 3 times mole of NOx is sprayed from pipe 7 by compressed air to be distributed uniformly in the cooler bank zone 3. Next, the gas passes through the gas cooling zone having sufficient number of heat conductive pipes to cool the gas to 300 to 430°C at the outlet of the cooling bank 3. Next, the gas is admitted into the cooling chamber 4, where the gas is maintained at such cooling temperature for a period of more than 0.1 second. Then the gas passes through the afterbank 5 and exhausted from the flue 6.