| 摘要 |
A venting aperture is formed in an upper wall of a casing, a gas-permeable membrane supporting body is mounted to the casing so as to communicate between the venting aperture and an external portion through a communicating aperture, and a gas-permeable membrane is disposed so as to shut off the communicating aperture. A cylindrical protective wall is disposed so as to protrude integrally from the casing so as to surround the gas-permeable membrane supporting body. A leading edge region of an external surface of the protective wall is formed so as to have an inclined surface in which a wall thickness gradually decreases from a root end toward a leading edge, and a groove having a groove direction in a direction of projection of the protective wall is formed so as to extend from a root of the protective wall to a leading edge and so as to communicate between an internal portion and an external portion of the protective wall.
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