To improve the durability of a metallic mesh while practically using the characteristics of the metallic mesh having excellent gas distribution function, and to prevent the gas distribution function and durability of the metallic mesh from being impaired even when a fluidized bed device is made large in scale.

The porous diffusion plate 13 is provided in the vicinity of the bottom part of a vessel 12 filled with the granular body, and is formed so that a fluidizing gas is passed and the granular body does not fall down. Further diffusion plate is constituted so as to blow up the fluidizing gas into the inside of the vessel through the diffusion plate from the under side of the diffusion plate. The enclosure 23 of the diffusion plate is fitted in the vicinity of the bottom of the vessel, many through holes 24a having diameter larger than that of the granular body are formed on the porous plate 24 integrally provided with the enclosure and the metallic mesh 26 provided at the upper side of the porous plate at the prescribed interval is formed so that the outer peripheral edge is fixed to the enclosure and the opening is made smaller than the diameter of the granular body.