PURPOSE: To complete the starting in a short time without requiring for excess capacity at the time of normal operation or more for a fluidized fluid supply system.

CONSTITUTION: The fluidized fluid supply unit 2 of the bottom of a furnace body 1 is divided into a plurality of compartments, and at the time of starting, the fluid is concentrically supplied to the partial compartment. Thus, the fluidized medium 3 of the compartment is raised at its temperature while fluidizing it. Thereafter, in the other compartment, the similar operations are sequentially conducted to raise the entire fluidized furnace at the temperature.