PURPOSE: To omit a filling seal material and filling work by inserting a waterproofing second annular packing between an insulating spacer and flange apart from a gas-sealing annular packing.

CONSTITUTION: A second annular packing 6 is inserted between the surfaces of an insulating spacer 1 and flange 2 and into a bolt 3 and cap nut 4. The insulating spacer 1 and flange 2 are fastened together by the bolt 3 and nut 4 for the purpose of enclosing the gas in a tank. In this case, a first annular packing is inserted between the surfaces of the insulating spacer 1 and flange 2. Apart from the first annular packing, however, the second annular packing 6 is arranged outward of the circumference, at which the bolt 3 is arranged. Thus, it is possible to prevent water from entering between the hole for the bolt passing through the insulating spacer 1 and fastening it and that bolt and to avoid the breakage of a spacer 1 effectively.