PROBLEM TO BE SOLVED: To increase a gasification speed and improve a processing efficiency by jetting fluidizing gas from a bottom part, throwing waste in a fluid bed in which a fluid medium flows to thermally decompose it and melting the produced thermally decomposed residue under the combustion heat of thermal decomposition gas. SOLUTION: A fluid medium 3a is fluidized by fluidizing gas jetted from a diffuser 2 provided on the bottom of a furnace to form a fluid bed 3. Then, when municipal waste is thrown in, the municipal waste is thermally decomposed in a fluid bed furnace 1. Thermally decomposed residue produced at this time is melted by a rotary melting furnace 20 using thermal decomposition gas produced in the fluid bed furnace 1 as a fuel and a part of exhaust gas after combustion is circulated as the fluidizing gas of the fluid bed furnace 1. Namely, high temperature exhaust gas generated in the melting furnace 20 is branched by a branch mechanism 17 via a first heat exchanger 14 and a second heat exchanger 15 for heating the fluid medium 3a. A part of the exhaust gas is supplied to a third heat exchanger 16 and then exhausted and a remaining part thereof is supplied to the diffuser 2 as the fluidizing gas.