PURPOSE: To lengthen starting time so as to prevent not to meet needs for shortening the starting time, by assembling a pressure vessel in such a manner that once bolts are completely loosed and tightened again by predetermines stress after permanent set is previously generated, and the bolts are tightened by the permanent set.

CONSTITUTION: Upper and lower flanges 6 integrated with vehicle chambers 1 which form a division surface, are integrated by tightening of tightening bolts 4 and tightening nuts 2 which are respectively inserted in respective opened holes and arranged on these flanges 6 so as to be assembled as a pressure vessel. When the washer down stress σw of the flanges 6 generated when the pressure vessel is started exceeds a yield point Y and reaches the stress σwx of a X point, permanent stress y is generated so that the lower part of a washer 3 is recessed. Therefrom stress is previously made to reach a stress application point X when the pressure vessel is assembled, and at this time, the permanent set y is simultaneously generated and the bolts 4 are tightened by the permanent set y, loosed and returned to a Z point, and after such a process, the bolts 4 are tightened again by predetermined stress and assembled, so that a lowering of tightening the bolts 4 can be prevented.