PURPOSE: To hasten the response of an NH3 flow control valve and to decrease control deviation by turning a tracking signal switching device ON when the change of signs of an operating signal of the degree of opening of the valve is detected, thereby tracking a command signal of the degree of opening of a valve by the degree of opening of the valve.

CONSTITUTION: Let the difference between an NOx concentration set value Ns prescribed by ordinances, etc., and an NOx concentration factor detected value Np from a plant be a deviation signal E. An operating signal 4h of the degree of opening of a valve is outputted from the deviation signal E and the NOx factor detected signal Nf by a denitration controller 8. When the operating signal 4h of the degree of opening of the valve having undergone a change to the different sign from the previous output is detected by a sign change detector 13, a tracking signal switching device 14 is made to be ON to give a command signal of the degree of opening of a valve tracking by the degree of opening (v). Therefore, overchute and underchute of an NH3 flow control valve 11 are eliminated to hasten the response, causing a rapid response to control.