To provide a control method of a combustion boiler capable of providing stable combustion without excessively increasing NOx concentration even in output runup and output fall.

In this combustion boiler comprising air supply pipe conduits 11a, 11b, an exhaust gas discharging pipe conduit 19, an exhaust gas recirculation pipe conduit 17 for recirculating a part of exhaust gas to the air supply pipe conduit, a GMF damper 16 disposed on the way of the exhaust gas recirculation pipe conduit for adjusting the amount of recirculation of the exhaust gas, an OFA damper 14 for adjusting the amount of air when a part of the air is injected from the wake of flame, a high-pressure turbine 31 for rotating the generated vapor of high temperature and high pressure, and a reheater 32 for reheating the vapor expanded by the high-pressure turbine 31, an opening of the OFA damper 14 is controlled by performing the correction on the basis of an air flow rate and an opening of the GMF damper 16.

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