<p>A worm gear mechanism is accommodated in a gear case (36) fixed on a yoke (24) and a control substrate (51) is fixed to the inner surface of a cover body attached to the gear case (36). A positioning unit (61) is provided in the gear case (36) and/or the cover body so that the positioning unit is positioned on the axis line (L1) of an amateur shaft (26), and the positioning unit (61) is inserted in a positioning hole (64) provided in the control substrate (51) to thereby position the control substrate (51). A sensor magnet (72) is fixed to the amateur shaft (26), and first and second holes (IC73a and IC73b) are provided in the control substrate (51) so that these holes (IC73a and IC73b) face the sensor magnet (72) and positioned away from the positioning unit (61) by the same distance. By these holes (IC73a and IC73b), the change in the magnetic field of the sensor magnet (72) is detected.</p>
