PURPOSE: To easily execute non-oxidizing heating by executing combustion with slightly excess fuel and using a waste combustion gas as a fluidizing gas after removing the moisture in this waste combustion gas.

CONSTITUTION: A heating chamber 5 and a retort 3 are heated by burning gaseous fuel. A fluidized bed (fluidizing particle materials: alumina, carbon, silicon carbide, etc.) using an inert gas as the fluidizing gas is formed. This fluidized layer is heated up by heat transmission between heated wall surfaces and the fluidized bed. The materials which are to be heated and are immersed in this bed are subjected to non-oxidizing heating in such a manner. The combustion by a burner 6 is, thereupon, executed with the slightly excessive fuel near the theoretical air-fuel ratio. The waste combustion gas is recovered and is used as the fluidizing gas after the moisture included in the waste combustion gas is removed. As a result, the base gas for fluidization having weak oxidability is formed at a low cost.