PURPOSE: To raise efficiency in completely gassified combustion of a granular solid fuel by providing a heating and carbonizing chamber with a grate on its lower end, a cylindrical primary combustion burner section with a plurality of air supply openings on its circumferential wall and a cylindrical secondary combustion section with a combustion products discharge opening.

CONSTITUTION: The oxidation and combustion are further promoted by the swirl wind from under a grate 4. A granular solid fuel that has burned on the grate 4 produces products in the state of combustion and oxidation, or of unburned gas and ash, etc. A swirl flow state is created in a heating and carbonizing chamber 5 by means of a screw device 23 for dispersing fuel, air supply tubes 3 jetted out to the grate, and the air from the grate 4, all working accumulatively. The multiple combustion layers of ignited fuel is introduced to a primary combustion burner section 7 via the clearances in the grate 4 successively from below and further promote combustion by the swirl air. Then also at a secondary combustion burner section 10 the effect of completely gassified combustion of combustion products is achieved with high efficiency with the aid of the same swirl air flow.