PURPOSE: To enable stable treatment to be made without causing blow-through despite the increase in the rate of supply by making the sectional area of the blow- up pipe supplying retreating medium gradually enlarge toward the ejection direction of fluid medium.

CONSTITUTION: Plural blow-up pipes 30 are opened in the slope of the bottom part of the main body near a swiveling porous plate 24. These blow-ip pipes are the tapered pipes of the shape in which the sectional area is gradually larger toward the ejection direction of fluid medium. Hence, the fluidized medium ascending in the pipes 30 by being blown by a blow-up gas gradually lower the velocities thereof with an increase in the sectional area of the pipes, thus joining smoothly to the fluidized bed 22 which is makin swirling motion. Thereby, the fluidized bed 22 is applied with the jet from the pipes 30 in addition to said swirling flow to make intricate flowing, so that the fluidized medium is well mixed and stirred and the fluidized medium is regenerated. Thence, the fluidized medium having undergone the regeneration is discharged through a pipe 27 and is supplied again to the reactor.