PURPOSE: To stabilze the temperature by lowering the value of the connecting resistance of a glow plug when either of the lowering of an added voltage or an electric source voltage or the engine cranking is detected after the transition to stable heating.

CONSTITUTION: At the time of engine cranking, since the heat of a glow plug is lowered, it is switched over to a dropping resistance 6 for preventing the lowering of temperature. When voltage of a battery 1 is too low, power is supplied only through the resistance 6, and a voltage near a rated voltage is added to the glow plug. When the battery voltage is too low and a starter is operated, resistances 5 and 6 are connected in parallel for raising the added voltage of the glow plug near to the rated voltage. This construction permits to stabilze the temperature of the glow plug.