A replay plate 44 is set to have a vertically and horizontally symmetrical shape when a worm wheel 33 is viewed from its axial direction; the worm wheel 33 is provided with an accommodating concave portion 33g in which the relay plate 44 is housed so as to be recessed in the axial direction; and a part of a non-slidably contacting surface in a slidably contacting surface 44a of the relay plate 44, i.e., a non-slidably contacting surface S is covered with first and second fixing parts 35a, 35b that are provided around the accommodating concave portion 33g and protrude in a direction intersecting with an axial direction of the worm wheel 33, the non-slidably contacting surface being a surface with which the contact plate is not slidably contacted.