Condensation water occurring within a container box (300) is discharged to the outside thereof, thereby keeping the inside of the container box (300) at a low humidity with no condensation water therein. The container box contains an electric circuit (210) therein and is sealed. The container box comprises: a minute discharge aperture (322) formed at a lowermost part of the container box (300); and a discharge valve (330), the discharge valve (330) being a check valve for closing the discharge aperture (322). Only when the an internal pressure of the container box (300) becomes higher than an external pressure thereof by a predetermined value or greater, does the discharge valve (330) opens to discharge the liquid present inside of the container box (300). The discharge valve closes to keep the airtightness of the container box (300) at all other times.