To simplify an apparatus and facilitate maintenance control by combusting a combustion product adjusting temperature to one where clinker is not produced, and performing heating fluidized particles with combusted gas, and further keeping an optimum the inside of a furnace by adjusting the amount of a fuel after furnace temperature reaches set temperature.

When heating upon starting is required, temperature of a combustion product is adjusted with an air ratio to a temperature where clinker is not produced and simultaneously combustion is performed with a burner 2 disposed at a lower part 3 of a furnace body, and with the combustion gas fluidized particles such as sand are flowed and simultaneously the fluid particles are heated. With the combustion, after the furnace temperature reaches set temperature, for example, 800°C, a combustion amount is mainly adjusted to keep optimum a fluidized state in the furnace and states such as fluized bed layer temperature. Herein, the burner 2 is set in its injection speed at least to 10m/sec or more. Further, the burner 2 is formed into a tapered shape, and there are disposed a plurality of burners tangentially on the same plane of the furnace.