To facilitate maintenance and to improve safety in operation and misoperation by mounting a means for closing a conductor end adjacent to a separation surface of a fitting box and a fitting plug.

Each joint casing forms an insulated vacuum chamber extended over the whole length of a fitting plug and a fitting box, and prevents the vaporization when a low-temperature medium flows in the conductor ends 12, 13. The conductor ends 12, 13 reach a separation surface of the fitting plug and the fitting box, and the means 28, 29 opposite to each other for opening and closing the conductor ends 12, 13 are mounted just adjacent to the separation surface. The means 28 of the joint box comprises a closing body for closing the conductor end 13, and loaded by the force of the spring and the medium to be contacted with a seal surface 33. Whereby the safety can be ensured in operation and misoperation.