To provide an apparatus for controlling the injecting rate of NH3 to an ammonia reduction denitrifying device which maintains an inlet NOx concn. within a set value by adequately controlling the injecting rate of the ammonia to the denitrifying device so as to match the reduction reaction of NOx and the cracking reaction of the NH3.

This apparatus has a means for calculating an inlet NOx quantity in accordance with the flow rate 103 of the exhaust gases to be treated and the inlet NOx concn. 101, a means for calculating a required molar ratio signal 106 in accordance with the set value 105 of the outlet NOx concn., a means for calculating a molar ratio correction signal 109 in accordance with the deviation 108 between the set value 105 of the outlet NOx concn. and the detected value 107 and a means for calculating the required NH3 flow rate 111 in accordance with these three signals. The apparatus is provided with a means for correcting the molar correction signal 109 by the output signals of function generators 1, 2, 3 respectively by functions f1, h0, h1 based on the inlet NOx quantity 104, the outlet concn. set value 105 and the change value 108 between the set value of the outlet NOx concn. and the detected value.