PURPOSE: To correctly detect the magnetic flux of a rotor magnet with a magnetic sensor by arranging the magnetic sensor out of the main direction of the magnetic flux of a stator coil so as to detect only magnetic flux leaking through the end faces perpendicular to the direction of magnetization of the rotor magnet.

CONSTITUTION: As shown by the figure, a hole element 31a is fastened to the inner flange 18a of an annular supporter 18 so as to detect magnetic flux leaking through the end faces perpendicular to the direction of magnetization of a rotor magnet 17. That is, the hole element 31a is close to the rotor magnet 17 and as distant from a stator 19 as possible to avoid the effect of magnetic flux generated by a stator coil. The magnetic sensor thus arranged prevents duplicated detection of the magnetic flux of the rotor magnet and that of the stator coil. Therefore, the magnetic flux of the stator coil has no effect on the magnetic sensor, the magnetic flux of the rotor magnet is detected accurately by the sensor, excitation of voltage applied to the motor is switched over with correct timing, and the excitation is controlled ideally.