PURPOSE: To suppress the transient increase in NOx at a load changing time, by a method wheren an inlet draft to each damper of primary gas and mixture gas is controlled uniformly, and reheated steam temperature is controlled through adjustment of the degree of an opening of an inlet damper.

CONSTITUTION: In a boiler which is constituted such that exhaust gas from a furnace 12 is recycled, the degree of an opening of an inlet damper 4 of a gas cycle fan 5 is regulated by an outlet draft, and flow rates of primary gas and gaseous mixture are held always constant. On one hand, the opening of an inlet damper 6 of a hopper 2 of the furnace is controlled to control the temp. of the reheated steam.