Abstract: The present invention is provided with: a casing (7) having an aperture (8); a protective panel (11) that is disposed within the casing (7) and that occludes the aperture (8); an o ring (12) disposed at one surface of the protective panel (11) in the direction perpendicular to the plane of the aperture (8); a sliding plate (13) that is disposed at the other surface of the protective panel (11) in the direction perpendicular to the plane of the aperture (8) and that holds the protective panel (11) in a manner so as to be slidable in the direction parallel to the plane of the aperture (8); a spring member (14) that is disposed contacting the edge of the protective panel (11) and that supports the protective panel (11) in a manner so as to be displaceable in the direction parallel to the plane of the aperture (8). The protective panel (11) is supported in the casing (7) by being sandwiched between the o ring (12) and the sliding plate (13).
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