PURPOSE: To prevent an excessive increasing of temperature of a gas bypass duct by a method wherein a gas bypass duct arranged in parallel with a boiler outlet duct and a cooling duct communicating with a discharged gas circulation flow passage are provided.

CONSTITUTION: A cooling duct 9 is arranged to communicate with a gas bypass duct 4 connected in parallel with an outlet duct 5 for discharging combustion discharged gas of a boiler 1 and supplying it to a denitration device and a discharged gas recirculation flow passage 12. When an inlet damper 8 of the bypass duct is closed, a cooling duct damper 10 is opened and a part of discharged gas of a discharged gas recirculation flow passage 12 of which pressure is increased by a gas recirculation fan 7 is passed through a cooling duct 9, supplied to the bypass duct 4 and then cooled. However, the cooling discharged gas is branched from an inlet of the denitration device 6, so that a gas temperature at an inlet port of the denitration device 6 is not decreased. The gas bypass duct 4 constantly passes discharge gas irrespective of a variation in load. Therefore, the accumulation of ashes can be prevented and a design load can also be reduced.