PURPOSE: To enable to improve heat efficiency and eliminate trouble of corrosion, etc. at heat exchanger tubes owing to ash adhesion on heat exchanger by burning completely volatile matter only and burning incompletely solid matter after woody fuel is separated into solid matter and volatile matter by burning in whirling motion.

CONSTITUTION: Coarse grain-like or compressively formed solid-like woody fuel is supplied into a combustion chamber 1 and is being burnt while the fuel is moved whirlingly downward in a combustion chamber space; thereby, the woody fuel is separated into solid matter and volatile matter and only the volatile matter is completely burnt to heat a heat exchanger 5 for working fluid and solid matter is burnt incompletely by being moved downward along the wall of the combustion chamber 1. Thereby, ashes are not produced in the combustion chamber 1 and moreover, burning solid matter is separated from air current and moved downward by centrifugal force and as a result, there is not trouble owing to ash adhesion on the heat exchanger 5 for working fluid, resulting in melting of ashes, etc.