The air dispersion plate at a lower part of a fluidized bed combustion furnace is provided with a large number of air injection nozzles uniformly distributed therethroughout and made of a porous refractory material having good air permeability and high heat and wear resistances. All of the fluidizing air supplied into the wind box below the dispersion plate is injected through these nozzles into the combustion chamber to fluidize the fluidizable material, which, after combustion is stopped, cannot flow through the nozzles into the wind box to cause heat damage and contamination therein and in the air supply pipe.