An inexpensive device for bus coupling between enclosed switchboards with an insulating gas sealed therein has a simple structure and requires a shorter time for installation. The bore shape of a connection port for a coupling bus in a T-shaped bushing is a conical shape having a diameter that increases outwardly. The coupling bus is covered with an insulating rubber member, and a conductive layer is provided on an outer peripheral surface of the insulating rubber member, and grounded. Both ends of the coupling bus that are inserted into the connection ports are formed to have shapes that fit to the conical connection ports.