PURPOSE: To perform denitrification with high efficiency in a denitrifying method wherein ammonia is injected into waste gas to convert NOx into harmless nitrogen by catalytic reduction by reducing the deterioration of denitrifying performance due to unbalance of NH3/NOx molar ratios on NH3 injection and restraining the increase of the quantity of unreacted leaked NH3.

CONSTITUTION: A denitrifying catalyst bed is divided in two or more to constitute a front flow side catalyst bed 6 and a rear flow side catalyst bed 7. In the front flow side catalyst bed 6, NH3 injection control for making an outlet NOx value constant is performed. In the rear flow side catalyst bed 7, NH3 injection control with constant NH3/NOx molar ratios for making an outlet NOx value the regulated value or less is performed. As a result, without a feedback control circuit of the outlet NOx value, denitrification with high efficiency to the NOx regulated value or less is performed and the quantity of unreacted leaked NH3 is lessened.