An oxyfuel boiler 1 is equipped with an oxygen separation apparatus 23 to produce oxygen by separating oxygen from the air, a recirculation fan 26 to supply a portion of combustion flue gas branched from the combustion flue gas discharged from the boiler to the oxyfuel boiler 1, a temperature sensor 14a to detect gas temperature at a boiler outlet 1d in the oxyfuel boiler 1, and a control apparatus 150 to calculate gas temperature at a furnace outlet 1c in the boiler from the gas temperature at the boiler outlet 1d detected by the temperature sensor 14a and control the operating condition of the recirculation fan 26 so that the calculated gas temperature at the furnace outlet 1c becomes a desired preset gas temperature.