PURPOSE: To prevent the temperature of glow plugs from being raised and lowered periodically, by feeding current to glow plugs only by a subsidiary feeder circuit having a register after the time when the glow plugs are heated to a reference temperature.

CONSTITUTION: Glow plugs 1 disposed in respective combustion chambers of a plurality of cylinders of a diesel engine are connected to a battery 5 via a main feeder circuit 4 including a current sensor 2 and a main relay 3 connected in series to each other, and a subsidiary feeder circuit including a register 6 and a subsidiary relay 7 connected in series to each other is connected in parallel to the main feeder circuit 4. When the glow plugs 1 are heated rapidly to a prescribed reference temperature by energizing the relays 3, 7 in starting the engine, only the main relay 3 is returned and kept in this state for a prescribed while. With such an arrangement, it is enabled to prevent the temperature of the glow plugs 1 from being lowered periodically, for instance, during cranking, so that startability of the engine is improved.