PURPOSE: To prevent the clogging of the orifice of a fluidizing grid by arranging a particle fall preventing means at a determined spacing from the bottom inlet of injection openings and constituting this means of a cup facing the bottom inlet of the injection opinions.

CONSTITUTION: The grid 2 for supporting the bed of the particles to be fluidized is provided with the plural injection openings 1 having the bottom inlet 3 communicating an injection gas supply box 5 and the particle fall preventing means placed between the bottom inlet 3 and the box 5 in order to hold the particles when the action is stopped. This particle fall preventing means is arranged at the determined spacing (i) from the bottom inlet 3 of the injection openings 1 and comprises the cup 5a facing the bottom inlet 1 of the injection openings 1. Consequently, the clogging of the orifice of the fluidizing grid may be prevented.