PURPOSE: To reduce the amount of waste gas by directly unloading an exhaust gas at one region of high-temperature primary air that has not been used up at the upper portion of an object to be burnt for introducing it to primary combustion air again and reducing the combustion air.

CONSTITUTION: A combustion space 14 is provided at the upper portion of a combustion fire lattice 5 and the front of the space 14 is terminated by a waste gas path 15. When an object to be burnt is burnt at the front of the fire lattice 5, the large part of primary air from a ventilation machine 12 is introduced via chambers 7-9. A waste gas in the combustion space 14 is unloaded via an exhaust path 24 that is connected to the suction side of a ventilation machine 25 from an exhaust opening 23 and is connected to the suction side of the primary air ventilation machine 12 via a compression side passage 26. Then, an exhaust waste gas is pressed to a passage 13 and is introduced to the chambers 7-11 as secondary air along with the primary air. Also, nozzles 21 and 22 are provided at the lower region of the waste gas path 15, secondary combustion air is introduced to the waste gas for burning a flammable constituent in the waste gas again, thus reducing the amount of waste gas.