To increase the efficiency of processing and relieve the influence on environment by flying ash of a fluid medium when waste is processed by fluidized bed combustion.

Waste is thermally decomposed by a fluidized bed 15 in a fluidized bed furnace 10, and thermal decomposition gas is moved through a gas moving passage 12. At a downstream area, the thermal decomposition gas is burnt at a low temperature by a heat exchanger 13 formed by combination of a catalyst filter carrying an oxidation catalyst and an air heater to effect heating of air. The heated air is introduced to the fluidized bed 15 through a heated air transfer passage 11 to maintain the fluidized bed at a high temperature. Thus, a substance excellent in fluid characteristics is selected as the fluid medium, and a substance optimum to a component of the thermal decomposition gas can be selected for the catalyst, and processing efficiency can be remarkably improved. Further, the mixture of the fluid medium in fly ash can be suppressed, and the influence on environment can be widely relieved.