PURPOSE: To facilitate the mutual positioning of mirror surfaces and to enable accurate, precise reproduction by providing a guide mechanism and a latch mechanism which have an accurate positioning function on the mirror surfaces.

CONSTITUTION: Outer periphery divided mirror surfaces 2 are assembled in order at the periphery of a center mirror surface 1 by being inserted toward the center as shown by arrows. The tip of a guide pin 3 which moves a mirror surface 2 to the mirror surface 1 as shown by an arrow is inserted first into the guide hole of the mirror surface 1. Then, the latch mechanism 7 is brought into contact with a latch metal fixture 8 and the mechanism 7 operates to couple the mirror surfaces 1 and 2. Then, an outer peripheral mirror surface 2b to be assembled is moved as shown by an arrow to the outer peripheral mirror surface 2a which is already fitted. A guide rod 9 which is sectioned circularly is guided to the V-shaped groove of the V-shaped fixture 10 almost simultaneously with the coupling between the mirror surfaces 1 and 2. Then the rod 9 and fixture 10 are brought into contact with each other to make the latch mechanism 11 and latch metal fixture 12 contact each other, and the mechanism 11 operates, completing the coupling.