To suppress the fluctuation of the main steam temperature of a boiler when a disturbance is given to the steam temperature of the boiler due to the soot blower of a reheater, a load change, etc.

The boiler is provided with gas distributing dampers 14 and 14 which respectively adjust the gas flow rates allotted to a primary superheater 4 and the reheater 10 positioned on the rear heat transfer wall of the boiler and a first spray control valve 12 for adjusting spraying flow rate which controls the outlet temperature of a secondary superheater 6. The difference between the outlet temperature of the superheater 6 and its set value and the difference between the outlet temperature of the reheater 10 used for controlling the opening of the dampers 14 and 14 and its set value are inputted to the control valve 12 as control signals.