PURPOSE: To contrive to decrease the generating rate of NOx considerably, by blowing combustion exhaust gas into a no-flame zone on the upstream side of a traveling-grate stoker in a combustion furnace from the under side of a grate, in a combustion furnace which burns solid fuel on a traveling-grate stoker.

CONSTITUTION: A fuel fed on a traveling-grate stoker 2 by a feeder 1 is burned by a combustion air 3' which is part of a combusiton air 3, and a generating gas 4 is discharged out of a furnace passing through a combustion furnace 5, with its temperature being decreased by a heat exchanger 6. At this time a recycling gas 10 which is part of a combustion gas 4 is blown off toward the no-flame zone on the upstream side of a traveling-grate stoker 2 from the under side of it. By circulating part of combustion exhaust gas and by blowing it off into the furnace in such a manner, generation of 10W20% of NOx can be prevented.