To eliminate inappropriate sampling and to evaluate the radioactive gas concn. as planned.

This vessel is provided with plural stop valves 14 and 16 and sealed by closing the vessel, and a pressure gage 22 indicating the pressure in the vessel main body 12 functioning as an ion chamber is attached to the main body. A Bourdon gage which is lightweight and highly durable is appropriately used as the pressure gage. The vessel main body itself functions as an ion chamber, the concn. of the radioactive gas in the main body is measured by connecting the main body to an electrometer, etc., and the pressure gage is utilized in the process to judge whether the main body is normally evacuated or not and whether the radioactive gas is normally sampled or not.