To permit the adjustment of an overflowing height during the operation of a fluidized bed.

The overflow pipe of a fluidized bed reaction furnace is constituted of an external overflow pipe 14, extending obliquely downward from a fluidized bed reaction furnace having a fluidized bed at the inside thereof, an internal overflow pipe 16, inserted into the external overflow pipe 14 so as to be movable into axial direction, and an overflow pipe moving device 18, moving the internal overflow pipe 16 into the axial direction thereof. The internal overflow pipe 16 is provided with a pair of rack gears 17a, extended into the axial direction of the external overflow pipe 14 and opposed to each other, and the overflow pipe moving device 18 is provided with pinion gears 19a, 19b, engaged with the rack gears 17a and turned into reverse directions mutually while the rotation of the pinion gears 19a, 19b is driven from outside by shafts 20a, 20b, penetrating the external overflow pipe 14 air-tightly.