PURPOSE: To reduce the amount of NOx effectively by controlling the amount of of a reducing agent to be added by the difference of gaseous pressure between the upper stream and down stream side of a heat exchanger on the upper stream side of a recirculating gas intake port of a waste gas flow passage, the gaseous temp. at the point for measuring the difference of gaseous pressure, and the concn. of NOx on the upper stream side of a denitration reactor.

CONSTITUTION: Differential pressure take-out seats 21 are provided on the upper stream and down stream sides of an economizer 7 and the pressure detection signals therefrom are inputted to an operator 24 via a pressure transducer 22. On the other hand, a gas sampler 19 connected to a gas analyzer 29 and a thermocouple 23 measuring gaseous temp. are provided to the down stream side of the economizer 7, and the NOx detection signal of the gas analyzer 19 and the temp. detection signal of the thermocouple 23 are inputted to the operator 24. The operator 24 operates these signals, and regulates the degree of opening of a regulating valve 19 by the control signal outputted from a controller 25, thereby controlling the amount of of NH3 to be added.