Because the pressure angle β° (=12°) of teeth (47) of a worm wheel (46) is larger than the pressure angle α° (=11°) of teeth (45) of a worm (44), tips of the teeth (47) of the worm wheel (46) can be more tapered as compared with a case in which the pressure angle of the teeth of the worm and the pressure angle of the teeth of the worm wheel are the same angle (the conventional case). Thereby, it is possible to ensure non-contact of teeth, which had been variable in terms of contact and non-contact in the case of the conventional form, even if there has been non-uniformity in the teeth (47) of the worm wheel (46).