PURPOSE: To compensate the phase of a feedback loop surely and stably while having said loop by connecting one end of a capacitor for phase compensation of the feedback loop to the output side of a detector and the other end to the output side of a comparator.

CONSTITUTION: The command current I0 of a voltage to current converter is supplied to a comparator 103. The output current of the comparator 103 is supplied to a distributing and feeding device 101. From the device 101, the current is supplied to respective phases of coils 12W14. The combined feed current to three phases of these coils 12W14 is detected as the voltage drop of the resistor 60 of a current detector 102 and is supplied to the comparator 103. As a result, a feedback loop is constituted of the comparator 103, the device 101 and the detector 102 so that the feed current to each phase of the coil attains the value corresponding to the current I0. Further in this case, a capacitor 110 for phase compensation is connected between the detector 102 and the comparator 103. Thus the phase compensation thereof is made stable and sure while having such feedback loop.