PURPOSE: To avoid the increase of a cost due to the increase of the number of parts by cutting off an exothermic resistor by the thermal deformation of a variable holding part and stopping the supply of a current to the exothermic resistor when the temperature runaway occurs in the exothermic resistor.

CONSTITUTION: A heating member 21 is arranged inside the ring of a ring-like heat-resistive belt 22, and the heating member 21 is provided with a planar heat generating body 21a for generating the heat by the supplied current and a holding member 21d for holding the planar heat generating body 21a on the variable holding part 21g which is deformed by heat at a prescribed temeperature. As to the heating member 21, when the temperature runaway occurs due to the trouble etc., of a temperature controlling system, the planar heat generating body 21a is cut-off by the thermal deformation of the variable holding part 21g and the current supply is stopped, so that the runaway of the temperature is automatically prevented. And also, the variable holding part 21g of the holding member 21d is formed of a separate attachable/ detachable member with refernece to a main body part 21e, and also, the thermal deformation characteristics of the part 21g is set lower than that of the main body part 21e.