PURPOSE: To control the generation of NOx by supplying solid fuel into a furnace after dust in solid fuel is previously removed when the solid fuel is burnt on a moving bed type fire grate.

CONSTITUTION: Dust container in fuel is separated by a dust separator 10, and the dust 11 is disposed of outside the system or blown in a gas combustion zone 8 in a furnace. The fuel from which the dust is removed is supplied to a combustion furnace 5 and burnt. By this constitution the generation of NOx can be reduced.