PURPOSE: To suppress the generation of a heat cycle between ordinary and very low temperatures requiring open/close sealing of a vacuum vessel during carriage to a using spot from the manufacturing plant, by coupling the vacuum vessel and coolant vessel using fasteners and enabling its fusing electric power to be supplied through a hermetic seal.

CONSTITUTION: A coolant vessel 2 is supported by adiabatic supporting members 3...3 and carriage-use fasteners 5, 5 inside a vacuum vessel 4. The adiabatic supporting members 3...3 have strength enough to support the coolant vessel 2 in a stationery state inside the vacuum vessel 4. Heating wires 8 are wound around respective fasteners 5, 5. These heating wires 8 are electrified by an outside power source 9 through terminals 7 continuing outside from a hermetic seal 6 mounted on a wall surface of the vacuum vessel 4. The device is composed so that the fasteners 5 can be fused by Joule heat generated in the heating wires 8 by this electric conduction.