PURPOSE: To lower the height of a free board by effectively collecting fluidized medium particles scattering or flying out from a fluidized bed.

CONSTITUTION: A large number of heat transfer pipes 21 for recovering the heat of exhaust gas are arranged to the free board 10 above a fluidized bed 8 side by side so as not only to provide intervals in an up-and-down and right and left directions but also to traverse the free board 10 and the filter 30 for collecting a fluidized medium 9 is allowed to come and go in the up-and-down direction at the attaching position of the heat transfer pipes 21 to be supported by the heat transfer pipes 21 and attached so as to traverse the free board 10.