PURPOSE: To efficiently perform action by variably setting a current present value, in which a switching element is turned off, in accordance with a DC power voltage value.

CONSTITUTION: A DC-DC converter 10 is constituted of a voltage converting transformer 15, switching element 17 for on-off turning a primary current of this transformer 15, rectifier circuit 19 of secondary side AC voltage and a peripheral circuit group for optimum-controlling the before described element 17, and a spark plug 35 is ignited through an energy accumulating capacitor 31 or the like. The peripheral circuit group is provided with a Zener diode 12, first to second comparators 13 to 14, circuits R1 to R3 for both dividing and adding power voltage and reference voltage and a voltage conversion detection resistor R4 or the like for the primary current, and a preset current value is relatively low set when a DC power voltage value is low generated.