An anterior wall of a case has a central aperture and two discharge apertures extending from the central aperture. There is an anterior chamber inside the anterior wall. Moisture such as rain water, even if entering the anterior chamber, is smoothly discharged out of the case from the discharge apertures along an inner face of the anterior chamber. As a result, it becomes possible to prevent the moisture from remaining and attaching to a first diaphragm in the anterior chamber and to prevent degradation of sound pressure collected by the first diaphragm through the central aperture and the discharge apertures.