PURPOSE: To eliminate the need of predicting a load so that a motor can be allowed to surely and relatively quickly rise by making the motor to rise by gradually increasing the power supply to the motor and detecting that the motor has risen from rotating period of the motor which reaches a prescribed period when the motor has risen.

CONSTITUTION: A CPU 14 detects the rotational frequency of a motor 11 by detecting the period of pulses from an encoder 13 and discriminates whether or not the speed of the motor exceeds an aimed low speed by discriminating whether or not the rotational period of the motor 11 exceeds a rotational period corresponding to a speed near the aimed low speed. When the CPU 14 discriminates that the speed of the motor 11 does not exceed the aimed low speed, because the rotational period of the motor 11 does not exceeds the rotational period corresponding to a speed near the aimed speed, the CPU 14 adds 2% to the Duty after waiting by 0.02 second. Therefore, the Duty having a initial value of '0' increases by 2% at every 0.02 second and the motor 11 rises as the power supply to the motor 11 gradually increases.