PURPOSE: To prevent the deterioration of a seal member, make a pressurizing means a small capacity, and improve safety by providing a pressure damping means damping the pressure on the cover section side in the seal device of a pressure container.

CONSTITUTION: When a cylinder device 38 as a pressure damping means damping the pressure on the pressure receiving face of a cover section 20 pressed by a cylinder device 46 as a pressurizing means is provided on a pressure container 4, the pressure applied to the cover section 20 is damped by the cylinder device 38, the pressurizing force to close the cover section 20 is reduced by the pressure damped quantity. As a result, the pressure applied to an O-ring 12 as a seal member installed between the cover section 20 and the pressure container 4 is mitigated, the force received from the cover section 20 is reduced to the degree required for sealing. The pressurizing force to close the cover section 20 is reduced by the damping of the pressure, thus the cylinder device 46 can be made a small capacity.