PURPOSE: To reduce the dispersion of the maximum value of coil currents by connecting a first fixed resistor and both a rheostat and a second fixed resistor in series and using the terminal voltage of the second fixed resistor as set voltage.

CONSTITUTION: A control circuit CNT, which employs reference voltage Vr set by a setting means S and detecting voltage Vs detected by a detecting means Rs as inputs and outputs a control signal driving switching means Tr1∼Tr4 in order to conform both current values, is provided. The setting means S is constituted of a first fixed resistor R1, a rheostat Rv and a second fixed resistor Rs connected in series between a power supply E and a ground while the terminal voltage Vr of the second fixed resistor is connected to a reference voltage input section for the control circuit.