A controller compares a minimum value of power source voltage which decreases in association with starting current generated upon start of a motor with a threshold value in a storing unit. When the minimum value is less than the threshold value, the controller notifies of shortage of the power source terminal voltage, and stops the motor. The controller also compares a value obtained by integrating the power source voltage which decreases in association with starting current generated upon start of the motor for only predetermined time with the threshold value in the storing unit, when the integral value is less than the threshold value, notifies of shortage of the power source terminal voltage, and stops the motor.