PURPOSE: To provide a cryogenic cooler in which the deterioration of the degree of vacuum of a vacuum layer is eliminated, an ultra-high vacuum is stably maintained for a long time, and a predetermined power can be reduced.

CONSTITUTION: An outer shell 13 is provided at the outside of a vacuum exhaust valve 6, and airtightly connected to a vacuum heat insulation outer tank 4 for a superconducting magnet by a metal sealing member 11. The part of the shell 13 is formed of a breaking means 8. The means 8 is formed of a breaking member 9 and a breaking plate destroying teeth 10. When the internal pressure of the shell becomes a predetermined value or more, the member 9 is brought into contact with the teeth 10 so that the member 9 is destroyed.