PURPOSE: To completely burn plastics contained in solid fuel.

CONSTITUTION: Solid fuel RDF particularly containing plastics is supplied from a supply port 7 onto a fuel feeder 9. The fuel is sequentially fed and burnt to the first, second and third zones 25A, 25B and 25C of a combustion chamber 25 via the feeder 9, and the ash is discharged from an ash discharging unit 15. In the zone 25A, the gas is fed from a communication hole 31 provided at a first partition plate 27 to the ceiling of the zone 25B at a relatively low temperature of 500°C or lower to burn the gas. In order to burn at a high temperature of about 800°C in the zone 25B, large quantities of primary and secondary combustion airs are fed from a plurality of diffusing nozzles to burn it. In the zone 25C, the primary and secondary combustion airs are similarly fed from diffusing nozzles 37 to burn solid. A second partition plate 29 is provided to expedite the neutralizing reaction of the combustion gas.