PURPOSE: To prevent a motor from running away due to the malfunction, such as a command current by limiting a current flowing to the motor when the motor is continuously rotated at a low speed.

CONSTITUTION: A speed command signal 8 from a motor controller 1 and a speed feedback signal 9 from a moving unit are applied to a servo amplifier 2, which amplifies its deviation to obtain a command current 10 for a motor 6. The current 10 is input to a motor driver 3. At this time, a motor power source controller 4 decides a power source voltage supplied to the motor 6 by a control signal 7 from the controller 1. Here, when the motor 6 is desired to be rotated at a high speed, the power source voltage supplied to the motor is raised so that a current limit is not effective, while when the motor 6 is desired to be rotated at a low speed, the power source voltage supplied to the motor is reduced to limit the current.