PURPOSE: To prevent abnormal increase of temperature of the heater by a method wherein a self-holding type solenoid relay is excited to intercept a current for a two-way three terminal thyristor by the output contact thereof when abnormality of temperature of the heater is detected.

CONSTITUTION: When the temperature of the heater 15 is normal, the output contact RY of an abnormality detecting relay 55 is being put ON and whereby the current is supplied to the two-way three terminal thyristor. When the detecting output of a temperature detecting element TS becomes an abnormal signal level, the reset coil RS of the abnormality detecting relay 55 is excited to put the output contact RY OFF and whereby the current for the thyristor is intercepted. When the trouble of short is generated in the two-way three terminal thyristor, which becomes the cause to increase the temperature of the heater 15 abnormally, the current for the thyristor is intercepted while the temperature of the heater 15 does not become abnormal and whereby the safe operation for the heater 15 may be realized.