A waterproofing device is provided for a mobile apparatus, using a thin coaxial cable or an optical cable and capable of being applied to a portable telephone of a bi-directionally openable/closable type. The waterproofing device is adapted to connect housings to each other by a linear member. The waterproofing device is provided with seal members consisting of a rubber-like elastic material and sealing a gap between the housings, and also with a connecting member having opposite ends in the vicinities of which the seal members are integrated. The linear member is passed through the inside of the connecting member to electrically interconnect electronic parts of the housings. The connecting member has a circular tube-like shape consisting of a soft resin material.