To use part of the air heated by a regenerative burner as fluidizing air and to simultaneously use the residual air as for combustion air of the burner.

In the case of alternately burning first and second regenerative burners 11, 12, the air is passed through thermal storage media 18, 19, preheated to high temperatures, and part of the air raised at its temperature is brought to a fluidized bed 14 via an air diffusing plate 15 and operated as fluidizing air. First and second exhaust air flow control valves 25, 26 are regulated to control an amount of supplying the preheated air to the bed 14 as the fluidizing air. Temperature of the fluidizing air is controlled according to a fuel supply amount and an air amount.