According to the present invention, by providing control whereby a rising slope or a descending slope of step-up current flowing to a step-up coil is detected, and corrections are made to step-up switching control, the step-up upper and lower limit current values of the step-up circuit can be controlled within intended current threshold values regardless of constant modifications or change in characteristics due to fluctuations of the battery power supply voltage or degradation of step-up circuit elements over time; heat emission by step-up circuit elements can be kept to a minimum; and the step-up recovery time can be adjusted to a constant value regardless of the slope of the step-up current.