A gear mechanism has a first gear which is coaxially connected to a drive shaft of a rotary driving source, a second gear which is meshed with the first gear, and a third gear which is meshed with the second gear and which is integrally interlocked with a ball screw nut. Diameters of the first to third gears are set to be smaller than a dimension of an upper body in a widthwise direction and a dimension of a lower body in the widthwise direction.