PURPOSE: To suppress an influence on a motor by a ripple contained in an output signal by installing a current controller which, according to a motor control command signal, changes the current flowing into a constant current device which determines the current flowing into a differential amplifier which amplifies an input signal.

CONSTITUTION: A base of a transistor for controlling current 30 is connected to a base of a transistor 14a, 14b of a current mirror 14 to make an amount of current flowing into the transistor 30 proportional to the current flowing into the current mirror 14 or an input signal T for controlling a torque of the motor. Therefore, when the input signal T is small, an amount of current flowing into the transistor 30 becomes small and as a result, an amount of current flowing into a transistor 18a becomes small and a gain of an error amplifier 10 is small. Eventually, an amplification factor to a ripple component contained in the input signal T becomes small and thereby a bad influence on a motor can be held down. When there is no input signal T, the amplifier 10 doen not work and the erroneous output does not appear.