PURPOSE: To develop a particular vacuum degree inside a cryostat by a method wherein absorbent is attached on parts where the temperature drops below a specified temperature, especially on cold heads.

CONSTITUTION: Absorbent 11 is attached on parts where the temperature drops below 100K such as cold heads of two-stage refrigerating machine 6 (1st stage 7 and second stage 8) and the external wall of a helium gas flow path, for example. Activated carbon, molecular sieve (made by BLD Co.Ltd.), etc., are used as absorbent. Air, hydrogen gas, helium gas, etc., are absorbed by the absorbent attached on the parts where the temperature drops below 100K, and the vacuum degree inside a cryostat can be maintained below about 10-6Torr for a long term.