A lens drive system includes a lens case supporting an image pick-up device; a movable lens frame supporting a lens group; a reversible motor supported by the lens case; a feed screw supported by the lens case and driven by the reversible motor; a half nut which moves together with the movable lens frame, is normally engaged with the feed screw, and is disengaged from the feed screw to slide thereon when more than a predetermined force is applied to the half nut in optical axis direction; and a plurality of ribs formed in the lens case projecting inwards behind a space for movement of the movable lens frame. Each rib includes a stop surface which comes into contact with a rear surface of the movable lens frame when the movable lens frame retreats while the half nut is sliding on the feed screw.