A curved portion, with a curved cross section, is formed at the recess of the pad. The curved portion has a contact portion which contacts the leaf spring's tension surface, which faces the curved portion. The curved portion's thickness decreases from the contact portion to an end portion of the curved portion, thereby being spaced from the leaf spring's tension surface. In the pad, the height of the curved portion of the recess is set such that contact of the leaf spring's tension surface and the corner portion of the end portion of the curved portion can be avoided when the stroke length of the leaf spring's end portion is at maximum. Thus, the tension surface of the leaf spring is not worn by the corner portion of the end portion of the curved portion. As a result, breakage of the leaf spring caused by wear can be prevented.