PURPOSE:To obtain a safe cryogenic cryostat having no freezing by a method wherein a safety valve, which allows only the stream of gas coming from an exhaust valve to flow in one direction, is provided at an arbitrary point on the side of an He vessel. CONSTITUTION:A safety valve 16 is attached to the exhaust hole 17 located between a flange 14 and an exhaust valve 15. If the valve 15 is closed when the liquid He is low in pressure, the valve 16 allowing to pass the unidirectioned air current only is closed, an airtight state is maintained, and outside air is not intruded into a liquid He vessel 23. When the pressure of the vessel 23 becomes high by the evaporation of the liquid He 22 remained in the vessel 23, the He passes the safety valve and flows to outside, and the vessel 23 is prevented from becoming high pressure. Also, the opening of the valve 15 is unnecessary. Accordingly, the safe cryogenic cryostat having no freezing can be obtained using a simple device.
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