PURPOSE: To decrease a magnetized ring width and an entire length as a motor actuator by providing to superpose a magnetizing phase for detecting a reference position Z phase of a motor rotation on a magnetizing phase for detecting a motor rotor position.

CONSTITUTION: In the case where signals of commutation sensors 2, 3 are LOW at the time of detecting a falling edge of a signal of a commutation sensor 1 by using a flip-flop, etc., a Z phase is input, and at the time of change of the signal of the sensor 1 to HIGH, an end of a Z-phase pulse is decided. In the case where the signals of the sensors 2, 3 are LOW at the time of detecting the edge, the signal of the sensor 1 is forcibly set to HIGH as a new signal of the sensor 1. Processes of the signals of the sensors 2, 3 are conducted similarly to that of the signal of the sensor 1 to exclude Z-phase signals from the signals of the sensors 1-3.