A guide roller projects from the side surface of an optical system body. The optical system body is held to be elevatable within a predetermined range along an arcuate guide member. A weight balancing means incorporating a spring attaches to the lower corner on the side surface of the optical system body. A hook attaches to one end of the arcuate guide member. A pulley attaches to the upper end of the arcuate guide member. One end of a wire fixes to the hook. The spring of the gravity balancing means pulls the wire through the pulley.
