PURPOSE: To perform rational denitrating control, by a method wherein a flow rate of a reducing material being injected is controlled at each turbine system so that a deviation between a desired value and a detected value corresponding thereto is brought to zero.

CONSTITUTION: A deviation signal (g) represents a deviation between a desired value at each turbine system and an actual value, and is inputted as a desired value signal for a flow rate of the injected reducing material to a subtractor 18 through a PI computer 15 and an upper limit value 16. An output signal (i) from a second detector 17 is inputted as an actual signal for a flow rate of the injected reducing material to the subtractor 18. From the two signals, the subtractor 18 computes a deviation between a desired value and an actual value, and a deviation signal (j) thereof is inputted to a PI computer 19. A flow rate of a reducing material, injected to each denitrating device through a regulator 20 for a flow rate of the injected reducing material, is controlled so that a deviation is brought to zero by means of an output signal (k) from the PI computer 19. Addition of a total amount control function to an individual control system enables the other control systems to compensate each other for the one control system which is happened to be forced into a condition in which it is deviated out of a control range.