PURPOSE: To enable to precisely continue preheating in case of the low power voltage of the preheating control device employing a digital computer at the start or the like by a method wherein the preheating control is performed by a back-up circuit.

CONSTITUTION: When battery voltage lowers, a detection circuit 16 for lowered voltage sets a signal S to "1". The flip-flop FF19 of a back-up circuit 18 is set by the signal S in order to set a signal S6 to "1". As a result, both the signals S7 and S8 of OR circuits 20 and 21 are set to "1", resulting in energizing glow plugs. When an engine puts itself in spontaneous running and its starting operation is completed and then a starter switch 3 is turned off, a starting signal S1 falls and FF19 is reset in order to end the energizing to the glow plugs. In such a manner as mentioned above, even when an operation circuit is put out of normal operation due to the abnormal lowering of the battery voltage, the normal preheating control can be performed.