PURPOSE: To modify output torque by modifying positive reference voltage, or obtain high torque at high speed by corresponding with increased voltage application.

CONSTITUTION: If a position detecting signal has been inputted through a terminal 4a, when a power source is thrown in, transistors 20a, 20b are conducted to start power supply to an armature coil M. Consequently, voltage drop occurs across a resistor 22 and when the voltage drop exceeds over the input voltage, i.e., a reference voltage 40, at (+) terminal of an operational amplifier 40a, output therefrom is brought to low level and the output from an AND gate 43a is also brought to low level, thus disabling the transistor 20a. Since the transistor 20b is conducting, magnetic energy stored in an armature coil K is discharged through a diode 21a and the resistor 22. Consequently, the transistor 20a is conducted again to increase armature current, and when the armature current exceeds a level regulated by the reference voltage 40 the transistor 20a is disabled again.