A central position detection section has a detection width in which the central position has a median value along one axial direction. When the direction from a position, which is located at the time at which returning is started toward the central position is defined as a go direction, the control section commands that a drive mechanism should sequentially perform following operations: a first operation of driving a projection lens assembly in the go direction; a second operation of allowing the projection lens assembly to travel in a return direction at least by the detection width; and a third operation of allowing the projection lens assembly to travel in the detection width in the go direction by ½ of the detection width traveled in the go direction.