In a position detector, a spring washer is beforehand attached integrally onto a carrier. Thus, when a planetary gear train unit composed of a sun gear, a ring gear and planetary gears is temporarily assembled, the center portions of the spring washer and a carrier are aligned each other accurately. Therefore, the axis of the spring washer cannot be misaligned from that of the carrier, the assembling performance is largely improved, and an accurate clutch holding force can be obtained. Alternatively, the spring washer may be attached integrally onto a cover plate.