PURPOSE: To suppress the overheating of an object and to prevent the damage of the object at the time of thermally decomposing such a material as plastic and removing the material from the metal object set in a furnace by controlling the object heating temp. in accordance with the output of a temp. sensor provided in a receiver at the lower part of the furnace.

CONSTITUTION: This furnace 2 is provided with a movable lower furnace 30 having a room 3, in which an object 4 to be treated is placed, and rotatable around hinges 31. An electric heating means 5 to be controlled by a controller 8 in accordance with the output of a temp. sensor 6 provided in a measuring space 7 demarcated by a receiver 9 below the object 4 is furnished in the room 3. Further, an air or oxygen feeder 15 is connected to the room 3, and oxygen is appropriately supplied from an oxygen source 20. A fixed upper furnace 32 separated from the room 3 by a wall 12 having an opening 13 and having a re-combustion chamber 11 with an opened outlet 14 is provided above the room 3, and a re-combustion device 21 and an oxygen feeder 23 are confronted with the re-combustion chamber 11.