PURPOSE: To produce a metallic mold at low cost and to surely intercept light advancing from a gap between a rotary shaft and a through hole into a dark box by making the bearing hole of the dark box as a through hole and integrally forming a cylindrical projecting part on the outside of the dark box so that it may surround the through hole.

CONSTITUTION: A projecting surface 22a covering the bearing holes 90 and 92 is integrally formed on the outer wall of a cartridge housing chamber 22. A projecting rib 50b engaging with the outer peripheryl of the projecting surface 22a is integrally formed with the dark box 50. Then, labyrinth structure is obtained by such engagement. Thus, the outside of the bearing holes 90 and 92 is covered by the projecting surface 22a. Since the projecting surface 22a and the projecting rib 50b are fitted by the labyrinth structure, the light is surely intercepted. Furthermore, the number of parts is not increased, whereby the cost is reduced.