PURPOSE: To increase denitration efficiency and that with reasonable H2O2 consumption in denitration of exhaust gas contg. NOx with reducing agent fed to the high-temp. atmosphere of the exhaust gas and with H2O2 fed to the low-temp. atmosphere of the exhaust gas, by adjusting H2O2 quantity to a specific mol ratio H2O2/NOx at the feed position.

CONSTITUTION: In order to denitrate exhaust combustion gas from a boiler, NH3 is added to the exhaust gas through headers 14a, 14b located at up-and down-stream sides of a secondary superheater 4 which is placed in high-temp. atmosphere of the exhaust gas. Then H2O2 quantity is adjusted so as to make the mol ratio H2O2/NOx come to nearly 1.5 at a H2O2 spray header 32 which is placed in low-temp. atmosphere of the exhaust gas of 500W600°C. The signal of analysis 24 of the exhaust gas delivered from a gas collector 33, and the signal of the exhasut gas quantity measured at a gas flow meter 34 are fed to a control box 25. The command given by the control box 25 controls simultaneously H2O2 flow control valve 36 and flow control valves 16, 16a for reducing agent NH3.