PURPOSE: To reduce NOx generation amount during low load operation of a boiler.

CONSTITUTION: A pressure detector 17 detects the pressures inside a furnace and outputs pressure detection signals 19, and a pressure detector 18 senses the pressures inside a window box and outputs pressure detection signals 20. A subtracter 21 obtains differential pressures 22 between the pressure detection signals 19 and 20. A function generator 24 obtains a differential pressure upper limit 25 based on a boiler load instruction 23, and a function generator 26 obtains a differential pressure lower limit 27 based on the boiler load instruction 23. A high-low monitor 27 checks whether the differential pressure 22 is out of a range determined by the differential pressure upper and lower limits 25 and 27, and transmits an opening/closing instruction 31 to a burner damper driver 30 when the differential pressure is out of the determined range.