To eliminate an incompletely burned state in a furnace body.

A water spraying nozzle 12 is mounted at a sidewall of a body 2 of a fluidized bed incinerating furnace 1 having a lower hearth 3 and its upper side secondary combustion chamber 7, and a water feed tube 13 provided with a flow regulating valve 19 is connected. A flow regulating valve controller 20 for controlling a valve travel of the valve 19 according to a difference between a hearth temperature T obtained by averaging detected values of a plurality of temperature detectors 16 by a calculator 21 and a set temperature T0 of a temperature setter 17 when the temperature T exceeds the temperature T0 is provided. Thus, the travel of the valve 19 is changed to continuously increase or decrease an amount of water 14 sprayed from a water spraying nozzle 12 to the hearth 3 to smoothly temperature change the chamber 7 in the case of controlling the temperature T.