To improve the sealing property of a high-pressure vessel while improving the convenience in taking in/out the high-pressure vessel by simple structure.

An opening 2A for entrance/exit is formed in the high-pressure vessel 2 for exposing a human body to the high-pressure environment, and a door 40 having larger dimensions than the opening 2A is disposed openably/closably on the inner side of the opening 2A. In addition, a tube 46 is set in the periphery of the door 40 to fill the gap between the opening 2A and the door 40 by expanding the tube itself with a gas.