PURPOSE: To improve an efficiency of a combustion by changing a dispersion method of a secondary air following to a decrease of a fuel, with a primary air is supplied upwardly from a lower part of a deposited layer of a solid fuel, and the secondary air is supplied being dispersed over upper and lower sides of the former.

CONSTITUTION: A primary air is introduced into a pipe line 22, a remainder of an exhaust gas and the primary air is supplied to a lower part of a combustion chamber 1 after being mixed. A control valve 52 is closed and a supply of a secondary air from a nozzle 49 is stopped, since the secondary air from the nozzle 45 is by-passed to an outlet port 13 of the exhaust gas, and an efficiency of a combustion is made worse, when an upper part of a diposited layer of a burning coke 3 is reached to a position 71, that is, the former is descended lower than a position of the nozzle 45 for the supply of the secondary air. Further, a control valve 53 is closed and the supply of the secondary air from a nozzle 50 is stopped, when the upper part of the deposited layer is reached to a position 72 owing to a progress of the combustion. Thereby, a stable and excellently effective combustion can be accomplished.