PURPOSE: To keep a stable fluidized bed by providing a cap material to the neighborhood of a gas inlet hole of fluidized bed cooler main body, and providing a line through which the extracted flid medium is circulated to said gas inlet hole side.

CONSTITUTION: A process gas is blown in from the clearance between a cap member 5 and a bottom part face of the fluidized bed cooler main body 1 via an inlet hole 1C from an inlet pipe 3 of the process gas, and the tar vapor contained in said gas is adhered to a fluid medium to separate and remove it from th gas. When supply of gas to gas to said cooler is stopped, the fluid medium is extracted via an extraction pipe 6 of the fluid medium from the main body 1 and is conveyed to the inlet hole 1C through a rotary valve 7 by gaseous nitrogen introduced from a nitrogen purge pipe 8. The conveyed fluid medium and gaseous nitrogen are blown in from the clearance between the cap member 5 and the bottom part face of the cooler main body 1.