PURPOSE: To accurately control a rotation and to reduce in size a motor by detecting an impedance change when the motor is rotated, and feeding back it to a controller of the motor.

CONSTITUTION: If a voltage is applied to the primary side of a high frequency voltage generator 4, a high frequency voltage is generated at its secondary side. This voltage is supplied to rotor winding of a commutator motor 2 through high frequency selecting means 3. A voltage between A and B proportional to an impedance change corresponding to a position where commutator segments pass a brush is present between detecting resistors 16. A voltage between C and B is optically converted by a photocoupler 20, and transmitted to a smoothing capacitor 22. A signal between D and E is input to one side terminal of a comparator 28, and compared with a reference voltage VTH. An output signal from the detector 5 is applied to a controller 7, and a rotation control is suitably performed through a motor driver 1.