PURPOSE: To heat solid fuel with usage of low energy by sending the low energy solid fuel into a secondary combustion chamber with an increased combustion air supply quantity after sending a predetermined quantity of the solid fuel into a primary combustion chamber and executing a primary combustion of the solid fuel with a decreased combustion air supply quantity.

CONSTITUTION: Combustion air is fed from a supply system 1, a solid fuel is supplied from a supply system 2, and these are mixed at a mixing portion 3. Besides, the solid fuel mixed with the combustion air is burnt in a primary combustion chamber 4. Further, a primarily burnt solid fuel is subjected to a secondary combustion in a secondary combustion chamber 6. In the above described composition, on the mixing portion 3, a combustion air supply quantity regulating portion 8 is auxiliarily mounted and on the solid fuel supplying system 2 a predetermined quantity of the solid fuel supplying portion 9 is auxiliarily mounted. And, after the predetermined quantity of the solid fuel is supplied into the primary combustion chamber 4, the combustion air supply quantity is decreased and a primary combustion is executed under a condition wherein the solid fuel is made to execute a residence in the primary combustion chamber 4. After that, the combustion air supply quantity is increased and the solid fuel is sent to the secondary combustion chamber 6.