PURPOSE: To keep pressure in a furnace constant, by compensating the time-lag of the action of a flue damper by controlling the deviation of furnace temperature, the primery factor of the fluctuation of furnace pressure, as one of flue damper drive elements.

CONSTITUTION: When deviation e is produced between furnace temperature detected means of a temperature detector 7 and furnace temperature set by means of a furnace temperature set instrument 10, the deviation is PD-operated by means of a computing element 12, furnace temperature signals I2 are put out, the signals I2 are added to furnace pressure signals I1, the output signals of a furnace pressure regulator 6, by means of an adder 13, and transmitted to an oil pressure cylinder 14 as furnace pressure control signals I and th opening of a flue damper 3 is adjusted. Consequently, this furnace is feed-forward-controlled using the deviation e of furnace temperature, the first factor of the variation of furnace pressure, as one of flue damper 3 drive elements, and the time-lag of the movement of the flue damper 3 is compensated by the commencement of the operation of the flue damper 3 at a stage that the deviation e is caused, thus improving the controllability of furnace pressure.