1 and n 2 fig3 is a view showing the driving circuit 101 of this invention in detail referring to fig3 the temperature controller 14 according to first embodiment will be described in detail as shown in the drawing an scr is arranged between the heating coil n 2 and the terminal n so as to switch on off the driving current for the heating coils n 1 and n 2 which flows through the terminal t during a temperature detecting cycle a temperature detector 31 detects a temperature voltage inducted to the temperature detecting terminal 12 arranged between the heating resistor r and the heating coil n 1 amplifies the detected voltage and outputs the amplified voltage to a temperature comparator 33 in a next heating cycle referring to fig3 a temperature setting unit 32 is installed to set a heating temperature of the heating element 20 this temperature setting unit 32 is realized as a variable resistor receiving a constant voltage vcc from a circuit voltage supplying unit 38 further the temperature setting unit 32 is arranged to operate in conjunction with a switch u201c sw u201d used for switching on off the driving current for the heating element the temperature comparator 33 compares a temperature or voltage detected by a temperature detector 31 during the heating cycle with the preset temperature or voltage outputs a u201c high u201d signal if the detected temperature is lower than the preset temperature and outputs a u201c low u201d signal if the detected temperature is higher than the preset temperature for power saving a zero detector 34 is installed in the temperature controller 14 the zero detector 34 detects a voltage at the terminal n generates a u201c high u201d signal for a predetermined period of time.