PURPOSE: To prevent tar from depositing and to increase the thickness of a fixed bed to enable a high-load combustion by agitating the fixed bed and supplying combustion air thereto.

CONSTITUTION: Supplied limes C are stacked on a lime layer C1 in burning at the lower part of a combustion thereby to form an unburnt coal layer C2, and a fixed bed is formed by lime layers C1 and C2. Air A1 is fed through a duct 19 to a furnace bottom 18, and also fed to the lower part of a combustion chamber through an air hole 39 of a grate 12. Air A2 which has been introduced into vanes 15 of an agitating rotor 14 is ejected through air ejection holes 16, and the lime C1 is burnt by air A1 and A2. Upon this occasion, the agitating rotor 14 is slowly rotated by means of a drive device 17, and coals on the fixed bed are agitated by vanes 15, thus improving the circulation of air and scraping off ashes adhering to the surfaces of coals to promote combustion. Further, air A2 blown off through the air ejection holes 16 of the vanes 15 prevents tar deposition generated when burnable components contained in a combustion gas G becomes surplus, and the quantity of heat necessary for a thermal decomposition is supplemented by oxygen contained in air A2 and a combustion reaction heat.