PURPOSE: To provide a combustor which is operable with a low oxygen concentration and emits clean exhaust gas by disposing an ash filter and a CO cleaning catalyst in a cleaning tube downstream of a combusting tube and providing a fan which circulates air to a space in the combustor outer tube.

CONSTITUTION: Combustion air is branched off at a junction 10 into a primary air pipe 11 and a secondary air pipe 12, and the branches enter into a combustor tube 1 from a grate 2 and a secondary combustion hole 13 respectively. The exhaust gas cleaned by an ash processing filter 16 and a CO cleaning catalyst 17 in a cleaning tube 14 flows through pipes in an air preheater 18 to give is heat away to combusting air, and, by so doing, it loses its temperature and enters into a cooler 19 situated at a high position to be cooled down to the outside temperature. The exhaust gas enters into scrubbers 20 filled with active charcoal etc. Said scrubbers are alternatingly operated to continuously remove excess CO2 from the exhaust gas. While one of them is adsorbing, the other releases adsorbed CO2 to the atmosphere by outside air blown in by a deadsorbing fan 21 to reactive the charcoal. The alternating operation of these two scrubbers 20 is controlled by the operation of valves 22.