PURPOSE: To adjust a height of bed of a fluidized bed optionally in accordance with a variation of a load for the fluidized bed and attain a stabilized combustion at all times by a method wherein a fluidizing medium reserving tank is arranged at a proximity of a fluidized bed furnace and both of them are connected so as to be capable of extracting and supplying the fluidizing medium.

CONSTITUTION: A fluidizing medium in a fluidized bed main body 2 is fluidized by air A supplied through an air chamber 9 and a perforated plate 10 and forms the fluidized bed 8. Fuel F is supplied to this fluidized bed 8 through a chute 11 and is burnt. In case a load is reduced under this combustion condition, a supplying amount of the fuel F is reduced at first, subsequently, a valve 12 on an extracting tube 3 is opened and a part of the fluidizing medium is extracted into a reserving tank 1 to reduce a height H of the bed. Due to this operation, heat transmitted to a steam pipe 14 is reduced and an evaporating amount is reduced. Thereafter, in case the evaporating amount of the steam pipe 14 is desired to be increased again, compressed air is sent through an air nozzle 6 to recirculate the fluidizing medium in the tank 1 into the main body 2 by the pressurized air stream and thus the height H of the bed may be increased.