PURPOSE: To slow rate of heating a glow plug and exothermic coil for preventing them from damages by connecting a first voltage lowering resistance in series to the exothermic coil of the glow plug through a switching means when the temperature of the glow plug reaches a preset value.

CONSTITUTION: Assuming that two glow plugs 1 are disconnected, two times voltage in the normal case are applied to the plug 1. When the composit resistance of the glow plug 1 exceeds a normal set value of resistance, a comparator C2 generates the output signal which is amplified by an amplifier A2 to operate a relay RL2 so that the normally closed contacts RL2 are opened. Next, a second glow plug heating circuit is opened and then a timer 3 generates the output signal after a set time of the timer 3 to operate a third relay RL3 so that the normally opened contacts rl3 are closed and thereby resistance R4 is inserted in parallel to resistance R2 forming a first bridge to halve the composite resistance. Thus, the glove plug 1 can be prevented from overheating.