A cryogenic system includes a containment vessel in which a body to be cooled is housed in such a manner that the body is immersed in a liquid coolant; a tubular refrigerator sleeve; and a refrigerator, inserted in the opening section of the refrigerator sleeve, for recondensing coolant gas generated from the coolant, wherein a gas flow-forming means for forming a flow of purge gas toward an opening section of the refrigerator sleeve is provided. The purge gas prevents air from entering a refrigerator sleeve during the replacement of a refrigerator of a cryogenic system.