To stabilize combustion irrespective of load fluctuations and attain high NOx reduction without causing a vibrating combustion in high load by adopting an NOx reduction system during high boiler load and an exhaust air recirculation system during low load time.

A plurality of reduction gas nozzles 7 are provided in the form of a ring on a furnace wall around a burner 6 where an NOx reduction fuel gas is arranged to be ejected (NOx reduction device). When boiler load is low and a furnace is at the temperature of about 800°C and below, the NOx reduction device fails to work. An exhaust air reflux flow rate control valve 13 is full- opened, thereby increasing the circulating amount of exhaust air (10 to 15% of combustion air). The value of NOx is about 300ppm in this case. On one hand, when boiler load is about 1/3 and over and the furnace is subjected to a high temperature, an exhaust air reflux flow rate control valve 13 is reduced (for example, it is reduced to 1/2 the full opening). The NOx reduction device works so that the reduction fuel may be ejected from the reduction nozzles 7.