PURPOSE: To shorten the preheating time by inserting a bridge circuit having a temperature variable resistor into the current supplying circuit of a preheating plug.

CONSTITUTION: A bridge circuit 4 is constituted of a first, second, third, and a fourth resistors 5, 6, 7, and 9, having the heating element 31 of a preheating plug 3 as one side. Through the insertion of a temperature-variable resistor (having positive temperature coefficient) in series to the second resistor 6, when the temperature due to the heating element 31 reaches a prescribed value after the turning-ON of a key switch 2, an output is generated in a comparator 10, and a starter 16 is driven by this output, and a relay 13 is opened through a delay apparatus 11. The bridge circuit operates so as to cut and permit the conduction to the heating element 31, and thus the melt of the heating element 31 of the preheating plug 3 can be prevented.