PURPOSE: To enable it to set a door's moving space and a storage space smaller and thereby reduce the cost of production by pressurizing a cavity part of the door at a closing position, pressing a peripheral edge of the door to an interval between opposed surfaces of a support member and an opening circumference, and sealing and closing them.

CONSTITUTION: When an opening 1 of a pressure vessel is sealed and closed by a door S, air is supplied to a cavity part 10 of the door S at a closing position via a vent pipe 11, thereby pressurizing the inside of the door S. With this pressurization, the door S expands to some extent, and a peripheral edge of the door S comes to be pressed to an interval between a peripheral edge of the opening 1 of a pressure vessel body A and a support member 2, thus it is relatively tightened. At this time, strength of the door itself and clamping force are increased according to pressurization in the cavity part 10, so that internal pressure in the door S is determined according to that in the pressure vessel. When a sealed state is set free, inversely air in the cavity part 10 is exhausted out of the vent pipe 11, and the door S is contracted. In this connection, air exhaust at this time may be done by dint of resilience of the door S itself and by means of connecting a suction pump or the like to this vent pipe 11 otherwise.