PURPOSE: To carry out an effective deodorization and denitration treatment by providing an NOx detector and a temp. detector at an outlet part of a heating furnace, an an anemometer and an anemoscope near an exhaust gas tower, and by controlling the control system of gas flow passages and of treatment condition of a reaction furnace by the detected values given by the above- mentioned instruments.

CONSTITUTION: The temp. of an exhaust gas is detected through a temp. detecting terminal 6 for the outlet part of heating furnace, and the operation of a denitration treatment is decided by a denitration correcting circuit 3. When a temp. of exhaust gas exceeds a set value and an NOx amount detected through an NOx detecting terminal 7 exceeds a set value of an NOx setting circuit 2, a closing command for a by-pass damper 28 of reaction tower an a starting command for an NH3 supply valve 29 and dampers 26, 27 for the inlet and the outlet of a reaction tower are issued from a denitration changeover circuit 4, and a denitration treatment is started. When a denitration treatment command is issued from a fixed direction and velocity setting circuit 1, the same denitration treatment as the above-mentioned is carried out.