To prevent generation of hexavalent chromium by not using a material containing chromium at a part possibly exposed to high temperature/flame.

A thermally decomposed matter from a thermal decomposition means 20 is introduced by carrying means 34 and 35 into a carbide combustion furnace 36 where the thermally decomposed matter is combusted and ashed. The combustion furnace 36 comprises a rotary furnace 37 internally provided with a refractory/heat resistant layer 38. On the side of the rotary furnace 37 for throwing in carbide, a burner 43 for combusting carbide is provided and a flame holding tube 53 composed of a metallic material containing no chromium is provided around the burner 43. The shoot part 35a of the carrying means 35 located in the vicinity of the combustion burner 43 is also composed of a metallic material containing no chromium. A discharge jacket 45 is provided on the ashing matter discharging side of the rotary furnace 37 and a stoker 46 provided in the discharge jacket 45 is composed of a steel material containing no chromium.