PURPOSE: To reduce a torque ripple by multiplying data calculated by measuring in advance the magnetic flux density of a motor and written in a ROM by a speed error signal to prevent an irregular torque from generating.

CONSTITUTION: The frequency of a detected pulse from an encoder 20 is converted by a frequency/voltage (F/V) converter 35 to a voltage, and a difference voltage between the voltage and a reference voltage generated by a reference voltage generator 36 is used as a speed error signal for a speed control loop. A motor 21 is driven by a signal produced by multiplying data calculated by measuring in advance the magnetic flux density of a motor and written in a read only memory ROM24 by the speed error signal.