PURPOSE: To perform a positive sensing of an amount of combustion air below a furnace for each of regions in a combustion chamber by a method wherein a degree of opining of each of dampers in a linear pipe is corrected with a coefficient calculated in reference to a pressure loss below the furnace for every region in the combustion chamber.

CONSTITUTION: At first, a flow rate of combustion air flowing in a main pipe 6 is calculated by a flow meter 9. In turn, each of pressures below the furnace of each of the regions DZ, CZ1, CZ2 and AZ is detected by pressure sensors 10A to 10D, the pressure loss in each of the regions is calculated in reference to its variation in time and further changed into coefficients (ak), (bk), (ck) and (dk) for correcting each of the degrees of opening (a), (b), (c) and (d) of each of the pressure loss dampers 7A to 7D. Each of these correction coefficients is fitted to each of desired equations and then an amount of combustion air below the furnace in each of the regions (DZ) to (AZ) is calculated in reference to a rate of degree of opening of the damper against a total amount of the corrected degree of opening of the damper and to an amount of combustion air flowing in the main pipe 6. As a result, it is possible to sense the positive amount of combustion air below the furnace in response to a variation of pressure loss when each of the stokers 3 is covered by the ignited material M.