PURPOSE: To prevent the decrease of degree of denitration by allowing a signal in a period of burner ignition or/and in a period of the changeover of a damper of a wind box to precede, actuating a signal injecting NH3 in the specified amount or more and performing a denitration reaction.

CONSTITUTION: An output signal is obtained from a multiplier 24 on the basis of both an NOx concn. signal 21 of a reactor inlet and a load signal 22 of a boiler. Also the relays 35, 36 are opened with the signals 33, 34 in a period of burner ignition or in a period of the changeover of a damper of a wind box and a signal sent from a signal generator 37 is added to an output signal sent from an adder 24 in an adder 40 via an adder 38 and a first-order lag setter 39 to use it as a set signal 25. Thereby NH3 is injected the specified amount or more in a period of burner ignition or in a period of the changeover of the damper of the wind box to prevent the decrease of degree of denitration.