PURPOSE: To avoid formation of a deposit and hence achieve economical cooling by forming a fluidized bed in an annular tank, introducing a high temperature process gas stream from a central duct of the fluidized bed, and providing a means carrying a cooled solid to a dust chamber on the fluidized bed.

CONSTITUTION: A fluidized bed 5 equipped with a cooling element 3 is formed in an annular tank 1, and through a gas permeable bottom fluid gas is then introduced into the fluidized bed 5. Further, a high temperature process gas flow 6 is introduced through a central duct 4 of the fluidized bed 5. A cooled solid overflows the annular tank 1 from the fluidized bed 5, and when it flows into the high temperature process gas flow 6, it is entrained to a dust chamber 21 on the fluidized bed flow 5. Meanwhile, the solid separated in the dust chamber 21 is returned to the fluidized bed 5 and is fallen down, and the cooled process gas involving a solid remaining part is introduced into a gas condenser 7 provided with a cooling surface 9. The gas exhausted from an upper portion of the gas condenser 7 is introduced into a dust removal apparatus 11, and the solid separated in the dust removal apparatus 11 is recirculated to the fluidized bed 5.