PURPOSE: To burn flue dust, which has so far been thrown away as unburnt loss, again and increase boiler efficiency by a method wherein unburnt carbon, included in exhaust gas, is collected to retrieve it and is blown into molten ash accumulated on the bottom of a furnace.

CONSTITUTION: Combustion gas of the burner 5 of a boiler 1 is sent to an air preheater 2 and a dust collector 3 as shown by arrow signs while smoke dust, including much amount of unburnt constituents in the combustion gas, is recovered here. The unburnt constituents, thus recovered, are adsorbed by molten ash 7, accumulated on the bottom of a furnace, through a reburning device 6, provided near the bottom of the furnace so as to be faced slightly downwardly with respect to a horizontal line, so that the unburnt constituents are not blown off by combustion gas, thereafter, the unburnt constituents are reburnt by utilizing radiation heat in the furnace.