PURPOSE: To attain an improvement in a fuel combustion efficiency, a thermal efficiency with respect to heat exchanging portions and durability of a combustion apparatus.

CONSTITUTION: A solid fuel F accumulated on a furnace bed 1 is caused to burn with a primary combustion air supplied from below the furnace bed 1, and a combustion smoke produced due to combustion is discharged from a flue gas outlet 5 opened near a ceiling 2b of a combustion chamber 2 provided on the furnace bed 1. The furnace bed 1 is of a substantially regular uneven construction 1a, 1b having slits, and an intermediate ceiling 6 with slits is provided in the combustion chamber 2 for vertically dividing the combustion chamber 2 into upper and lower halves.