PURPOSE: To provide both performances of control for maintaining a specified denitration reduction ratio and control for maintaining specified NOx in the inlet of a chimney and to enable automatic changing over of both by adding the control signal for maintaining the specified NOx value in the inlet of the chimney to the signal from a reduction ratio setter.

CONSTITUTION: The total NOx amt. calculated by a multiplier 11 from the output signals of a signal transmitter 9 for the flow rate of air for combustion and a detector for NOx in the gas at the outlet of a fuel economizer and the signal from a reduction ratio setter 13 are calculated with a multiplier 12, and the output thereof is used as a demand signal for the amt. of gaseous NH3 for denitration in controlling NOx at the specified denitration reduction ratio. On the other hand, the signal of a detector 24 for NOx at the inlet of a chimney and the set signal of the NOx value at the inlet of the chimney are calculated with a calculator 26, and the deviation thereof is inputted through a multiplier 27 which adjusts the gain so as to meet load to a proportional plus integral calculator 28. The output of the calculator 28 is inputted to an adder 28 by which said output is added to the signal from the setter 13 and is converted to the set value for specified denitration reduction ratio from optional partial load, whereby the control for maintaining the specified NOx value at the inlet of the chimney is accomplished.