PURPOSE: To prevent thermal distortion due to welding by jointing a pressure- proof vessel to a linkage member through a seal member by bolting.

CONSTITUTION: The linkage member 4 is tightened with a bolt 4 through the front direction pressure-proof vessel 2, the rear direction pressure-proof vessel 3 and the seal member 13 so as to be jointed. At both ends of the linkage member 4 circular projections 15 are fitted, which facilitate positioning at the time of bolting said vessels 2 and 3. Moreover, an I/O port 15 for taking in and out a cable which transmits information from a mother ship is installed on the linkage member 4, and therefore the production of a pressure-proof vessel is facilitated and the reduction of its strength can be prevented.