To prevent the damage to a heater by lifting a heating tube or the heater from a melting vessel immediately at the time when the insulation of the heating tube is destroyed.

A heating tube 30 comprises a protective pipe 14 provided with an electrically conductive layer 14a inside or on the inner wall surface and a heater 18 comprising a heating wire 16 arranged inside the protective pipe 14. There are provided a continuity sensor 24 detecting the electric continuity between the electrically conductive layer 14a and molten metal 17 and a lifting mechanism 27 lifting the heater 18 from a melting vessel. The lifting mechanism 27 is driven by the output signal of an electronic control unit 25 on the basis of the signal of the continuity sensor 24.