PURPOSE: To permit to burn efficiently various wooden solid fuel by a same device by a method wherein air is supplied into a combustion chamber from a lower part, a side part and an upper part.

CONSTITUTION: The proportion of the air supplied to a combustion chamber 2 through respective ventilating parts 5W7 of a lower part, a side part and an upper part from a ventilating device 8 is controlled optionally by the controlling of respective choking devices 4, 4, 4. This air is supplied to the combustion chamber 2, after primery heating during passing between inner walls 12, 15 and outer walls 16, 19 of the combustion chamber 2. The fuel supplied to the combustion chamber 2 is dried on the way of sliding down to a valley side from a mountain side of a furnace bed 11 and burnt when it arrives at the surface of lower slits 10, then, a flame 21 is generated and extended to right side by air pressure comming from aslant upper direction and aslant lower direction, therefor, the air supplied for the flame 21 from the side part and upper part, therefor, the fuel is burnt completely. Heated air generated in this combustion is discharged out of a chimney 23 through a heat exchanger 22 from a flame path 9.