PURPOSE: To create a high temperature atmosphere having a uniform temperature distribution by providing a means for jetting air toward the center of a combustion chamber in a substantially horizontal direction, below a gas burner whose nozzle is directed toward the center of the combustion chamber, so as to guide flame from the gas burner toward the center of the combustion chamber under action of the stream of jetted air.

CONSTITUTION: Air press-fed to an air charge chamber 54 from a flue gas fun 42 is jetted into a combustion chamber 18 through a plurality of air jet outlet ports 62 which are formed in a part of an inner wall 14 that defines an air charge chamber 54, below the level of a gas distribution pipe 46. Since this air contains fresh air taken through a center opening in a top plate 30, it contributes to the combustion of gas fuel discharged from the gas distribution pipe 46. Flame 64 from a gas burner is elongated in a direction toward the center of the combustion chamber 18, realizing a high temperature atmosphere having a uniform temperature distribution in the combustion chamber. A part or all of smoke sucked by a flue gas device is returned into the combustion chamber 18, and accordingly, fat components may subjected to after-burning treatment.