A rotary shaft is formed with an external screw thread; a movable nut is engaged with the rotary shaft, and arranged to move axially in accordance with rotation of the rotary shaft; and a link member is swingably connected with the movable nut to transmit motion from the movable nut. The link member includes first and second side portions and a connecting portion connecting the first and second side portions and forming a depressed portion in which the rotary shaft is received when the link member is in a predetermined leaning posture.