Minimum fluidization voidage in the beds of three kinds of silica is investigated at elevated temperatures up to 1223 K experimentally and theoretically. The bed voidage depends not only upon the operating temperature but also upon the humidity of fluidizing gas. Experimental results and theoretical consideration on the change in the bed voidage with temperature and/or humidity demonstrate that this change is caused by the adhesion force of silica particles originating from the change in the concentration of siloxane groups on the surface of the particles.