To treat dioxins in a fluidized bed type incinerator and to reduce construction costs and maintenance management costs of facilities.

In a furnace structure, a container bearing structural strength of a furnace has a circular horizontal section and utilizes merits of a cylindrical shape advantageous for strength and thermal loss, a hearth of a fluidized bed part is an inclined plane shape inclined downward to an incombustible discharge port and a horizontal face shape of the hearth, which is brought into contact with at least the incombustible discharge port, and a horizontal section shape of a fluid medium inversion wall located on an upper part of the hearth are aligned. A flow direction of an inner circulation flow is set to a direction rectangular to the fluid medium inversion wall and the fluidized bed succeeds merits of the rectangular inner circulation flow to obtain slow gasification/combustion of waste. Then, a generation area of dioxins is narrowed, a sum total of combustion air quantity for decomposing dioxins and air quantity to be supplied to the fluidized bed part is set to 100% or more of logical combustion air quantity to the whole objects to be combusted, and dioxins and their precursors are efficiently decomposed and combusted by a sufficiently large oxygen quantity supplied to a place of a high concentration state of dioxins not diffused just after generation of dioxins and a high temperature. Consequently, construction costs and maintenance managements costs of facilities can be reduced.