A pressure vessel includes: an outer container unit having an inner space and first and second supporting portions; a lining container disposed inside the outer container unit and having an end portion that corresponds to the first supporting portion; an elastic diaphragm disposed in and dividing the inner space into air and water chambers, and having retaining and extending sectors that correspond in position to the end portion and the second supporting portion respectively; and a pressing member disposed in the air chamber and having first and second pressing segments that press tightly the retaining sector and the end portion against the first supporting portion and the extending sector on the second supporting portion respectively.