PURPOSE: To maintain a weakly reducing atmosphere in a fluidized bed furnace to be used for forging, heating, etc., of steel materials and to prevent oxidation of the steel materials to be heated by using the combustion gas of a burner in the furnace as a gas for forming a fluidized bed, returning a part thereof to the fluidized bed furnace and adding a combustible gas thereto to burn the above-mentioned gas over the fluidized bed.

CONSTITUTION: The fluid sand on a diffusion plate 10 in the fluidized bed furnace 1 is fluidized by the combustion gas blown from a lower inlet 15 and the steel materials are put therein and are heated to a prescribed temp. A waste gas filters the fluid sand by a filter 3 and a part thereof is taken from a branch line 6. After the residual fluid sand is completely recovered from the waste gas by a cyclone 7, the gas is returned again to the fluidized bed furnace 1 by a heat resistant turbo pump 8. The content of O2 in the waste gas is detected by a sensor 21 and the quantity of the air to be supplied to the burner 2 above the fluidized bed is regulated. The content of the O2 in the fluidized bed is simultaneously detected by sensor 11 and a control valve 13 of a gaseous propane cylinder 14 is controlled to provide the weak reducing property to the waste gas to be supplied to the fluidized bed so that the oxidation of the steel materials is prevented. Since the waste gas from the fluidized bed contains propane, etc., the air from the burner is fed thereto to burn the waste gas. This gas is recirculated as the high-temp. waste gas and the sensible heat thereof is recovered.