PURPOSE: To prevent a motor from being brought to a locking state by increasing the quantity of conduction to the motor before the motor is locked when load is applied too much and the number of revolution is lowered.

CONSTITUTION: The number of revolution of a motor 13 is measured by a revolution detecting circuit 102 containing a photo-interrupter 16, a transistor 17, a capacitor 18 and a comparator 19, and the lowering of the number of revolution is detected. A gate-pulse generating circuit 103 including a capacitor 20, a transistor 21, a diode 22 and a resistor 23 generates a gate signal for conducting a thyristor 14 at all times when the lowering of the number of revolution of the motor 13 is detected by the revolution detecting circuit 102, thus increasing the quantity of conduction to the motor 13.