PURPOSE: To maintain a boiler operation pattern in its optimum condition by setting the temperature of exhaust gas mixed after it has passed an air preheater and an exhaust gas cooler respectively as a minimum value and controlling the flow rate of circulation water in response to the concentration of SO2 with a flow rate control valve.

CONSTITUTION: An exhaust gas-contained SO2 meter 12 is adapted to measure the exhaust gas of a boiler 1 which uses a blast furnace gas(BFG) as a fuel, which flows past a combustion air preheater 2 and an exhaust gas cooler 3 and is sucked and mixed by an induction fan 8. A circulation water low temperature side thermometer 11 is adapted to measure the temperature of circulation water of a BFG heater 9. The measurement data enters a control device 24 together with the signals of the SO2 meter, a BFG flow meter 22 and a circulation water flow meter 23. A flow rate control valve 18 of circulation water is adjusted so that the temperature of the exhaust gas may be minimized in conformity with the concentration of SO2. It is, therefore, possible to prevent the corrosion of an element at a low temperature portion, reduce the temperature of the exhaust gas and preserve high heat with efficiency.