PURPOSE: To conserve the electric power to drive a blower, by detecting the temperature in a combustion furnace and the density of oxygen contained in an exhaust gas, and by controlling the rate of air to be fed by compensating a temperature signal by an oxygen density signal, as well as to control the rate of fuel by a temperature detecting signal.

CONSTITUTION: The temperature in a boiler 3 is indicated by a pressure detected by a pressure detector 4 corresponding to the temperature in a boiler, and the rate of fuel to be fed into a furnace 1 is controlled such that the opening degrees of a control valve 13 is controlled by a signal which controls the rate of fuel from a controller 5, which is produced by the difference between the signal of a pressure detector 4 in regard to the temperature in a combustion furnace 1 and the set value. On the other hand, the output signal from a controller 5 is compensated and calculated in a calculator 9, in order to determine the output signal from an oxygen detector 6 to the predetermined density, in regard to the density of oxygen contained in an exhaust gas from a furnace 1, in response to the signal which is sample-held by a sample-holding circuit 7. Then the rate of draft from a blower 12 is controlled according to the rotation of a motor 11 by the intermediary of a manipulator 10.