| 摘要 |
PROBLEM TO BE SOLVED: To provide a mirror tunnel unit on which external light is not made incident from an interstice between a negative mask upper surface and the mirror tunnel unit even in the case that the thickness of a negative mask is changed according to the kind of it. SOLUTION: A light shielding cover 16 to cover the entire periphery of the lower end of the negative mask side attaching part 15 of the mirror tunnel unit 12 is freely attachably and detachably attached. The light shielding cover 16 is made freely movable in the axial direction of the negative mask side attaching part 15, so that the lower surface of the light shielding cover 16 is made in a state where it is always placed on the upper surface of the negative mask 17. Thus, the light shielding cover 16 is always placed on the upper surface of the negative mask 17 even in the case that the thickness of the negative mask 17 differs according to the kind of the negative mask 17, so that it covers the lower end periphery of the negative mask side attaching part 15. Then, though the interstice is caused between the lower end of the negative mask side attaching part 15 and the upper surface of the negative mask 17, the external light from the interstice is stopped from being made incident inward.
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