PURPOSE: To enable effective reduction of production of NOx, by a method wherein a part of combustion exhaust gas is bled to feed the combustion exhaust gas in a position situated above a spot in the vicinity of the upper end of flame formed on a stoker.

CONSTITUTION: A part of combustion exhaust gas after it passes a heat exchanger 6 is bled through a recirculating gas line 10, and is fed to portion above a spot in the vicinity of the upper end of flame 8 formed on a moving stoker 2. As noted above, by blowing in combustion exhaust gas instead of air for combustion, oxygen concentration in a combustion area is reduced, and simultaneously, by blowing the exhaust gas in a furnace, mixing of oxygen and unburnt gas in a furnace is promoted to enable execution of low O2 combustion. This constitution prevents production of high NOx.