PURPOSE: To maintain drive voltage of a fuel pump in constant by controllably varying, in response to the variation of supply voltage, the terminal voltage across a capacitor of a motor fuel pump connected to a power supply through the capacitor in series.

CONSTITUTION: When supply voltage Vb of an alternator or the like is varied, for example, boosted, the corresponding reference voltage V0 of a reference voltage setting circuit D obtained from the potential dividing ratio is also boosted. As a result, the output of a comparator A1 becomes "L" to increase OFF time of a transistor T5 and the charge amount of a capacitor C2 so that the terminal voltage is boosted by the boosting of the reference voltage. Also, when Vb is lowered, the reference voltage V0 is lowered and accordingly the output of the comparator A1 becomes "H" so that the terminal voltage of the capacitor C2 is also lowered by the fall of the reference voltage V0. Thus, the drive voltage of a fuel pump can be maintained in constant.