PURPOSE: To enhance treating efficiency by a method wherein a passage, the sectional area of which is made to reduce upward, is formed by quide walls which are provided within a bed, and a fluidized bed is provided in a part having optimum sectional area within the passage depending on the amount of fluidizing gas to always fluidize a material tube treated uniformly.

CONSTITUTION: A distributor 2 for fluidizing gas is provided within a bed 7 in the lower part of the main body wall 1 of a fluidized bed apparatus. Above the distributor there are provided two guide walls 3a, 3b opposite each other. The two guide walls are arranged so that the space between two walls becomes narrower upward and wider downward to provide a reduced passage 4 the sectional area which is made to reduce upward. Between the reduced passage 4 and the main body walls there are provided circulating paths 5a, 5b the sectional area of which are made to increase upward. A fluidizing gas B and a material C to be treated are supplied from the bottom to be apparatus. The material C ascends together with the gas B within the passage 4. The gas B and fluidized materials Ca mixture of fluidizing medium and the material C to be treated) are gradually accelerated within the passage 4. Thus, optimum fluidizing velocity can be obtained within the passage 4, and uniform fluidized layer D is formed naturally.