To prevent a central portion of a retort from deflecting downwardly thus preventing an occurrence of an excessive stress by dividing a furnace body into a plurality of furnace bodies in a spaced-apart manner and providing an intermediate support device between neighboring furnace bodies for rotatably and freely supporting the retort.

When a raw material is charged into a first furnace body 2 through a raw material charging opening 9, the raw material is forcibly agitated by agitating blades 11 and heated and organic materials are burnt. Gas and vapor generated in a retort 1 are discharged into the furnace body 2 by way of an exhaust pipe 8 by holding the inside of the retort 1 at a temperature higher than the inside of the furnace body 2. The similar treatment is carried out in a second furnace body 3 and a treated material is discharged from a discharging opening 10. Since the retort 1 has not only both ends thereof but also a central portion thereof supported by a roller (an intermediate support device) 14, even when the retort 1 is extremely elongated, the central portion the retort 1 is prevented from deflecting downwardly thus there is no fear of rupture caused by a rotational bending under high temperature.