PURPOSE: To permit the maintenance of the optimum condition of fluidized combustion at all times by constituting the incinerating furnace so as to discharge unburnt matters on a dispersing plate quickly when fluidizing has become inactive.

CONSTITUTION: A fluidized bed incinerating furnace is equipped with a bottom pressure sensor 51 installed immediately above a dispersing plate 24 provided reciprocally, an upper pressure sensor 52, installed at the upper part in a furnace, a deciding device 54, receiving the signals of both pressure sensors 51, 52 and effecting a predetermined operation to judge the condition of fluidizing, and a driving device 55, reciprocating and driving the dispersing plate 24 so that unburnt matters on the dispersing plate 24 are moved toward the side of a discharging port 28.