PURPOSE: To conduct a favorable stall compensation even if an acceleration/ deceleration rate is varied by frequency reference compensating only with a stall compensation signal during stalling.

CONSTITUTION: A frequency reference circuit 31 determines a frequency reference according to the target set value of a frequency and a stall compensation signal. An input limiter 41 inputs the target set value of a speed setter 8, increases or decreases it at a predetermined ratio with respect to a time to generate a frequency target signal, and stops the increase or decrease by a stall detection signal. An adder 35 adds the output of the limiter 41 and the stall compensation signal input through a contact 34, and outputs a frequency reference. Thus, if a stall compensator is operated, it limits the variation of the target set value of the frequency to stop the variation of the target frequency signal, and compensates with the frequency reference only by the stall compensation signal during stalling.