PURPOSE: To improve the energy saving and the air-tightness by covering a vacuum vessel and a high-frequency induction heating coil with an attachable/ detachable heat insulating cover and cutting off the heat conducted to an air- tight seal part with heat cut-off mechanism.

CONSTITUTION: Parts 9 to be treated is put in an exothermic body 5. The inner part of the vacuum chamber 1 is evacuated. Current is conducted to the coil 4 for high-frequency induction heating to heat the exothermic body 5. Since the heat insulating cover part 6 covers the vacuum chamber 1 and the coil 4 for heating, the energy saving is achieved. The heat insulating cover part 6 preferably adopts a material excellent in heat insulation, large in heat reflection and small in heat radiation, such as heat-resistant ceramics. In the cooling process, the cover of the heat insulating cover part 6 is taken off to quickly cool the exothermic body 5. The conduction of the heat to the air-tight seal part 3 is prevented with the heat cut-off material 7 and the deterioration of the sealness is prevented.