To simplify a lens exchange by providing a lens end face with a hole and providing this hole with a pin projectable to a bracket (or bridge).

The detaining pin of the bridge is inserted into the small hole 32 of the lens 30 and the detaining pin 11 of the bracket 10 is put into the submerged state by pulling the projecting part 11 of the bracket 10 with the fingernail from the state that the detecting pin 11 is not yet inserted. Next the lens 30 is pushed between branched browbars 40 and is positioned in correspondence to the position of the detaining pin 11 in the submerged state. At this time, the lens 30 is held pinched by the browbars 40. The fingernail is released from the projecting part 111 and the detaining pin 11 is inserted into the small hole 31 of the lens 30 projected again by energizing force. The mounting of the lens 30 is completed in such a manner. The removing of the lens 30 from the bracket 10 or the bridge 20 is executed simply by following the procedures reverse.