to have a ring shape may be employed instead of the above sealing ring when a cold corresponding to a very low temperature is generated at a lower end of the second stage expansion chamber 14 using the cryogenic refrigerator the cryogenic refrigerator employing sealing rings made of beryllium copper having the above arrangement and when a squid not illustrated is in a superconducting condition by the generated cold and is operated a frequency characteristic of an outer magnetic field was a characteristic illustrated in fig4 on the other hand when a squid is operated using a cryogenic refrigerator which employs sealing rings having the arrangement illustrated in fig2 and 3 made of stainless steel instead of sealing rings made of beryllium copper a frequency characteristic of an outer magnetic field was a characteristic illustrated in fig5 as is apparent from both figures a white noise level was 27 5 ft and a peak of 2 hz noise was 0 77pt in the case illustrated in fig4 on the contrary a white noise level was 42ft and a peak of 2 hz noise was 1 3pt therefore a peak of 2 hz noise which gives great influence to magnetocardiogram measurement and the like is reduced by approximately 40 and it is understood that a magnetic noise and disturbance of magnetic field are greatly reduced by changing the material of the sealing ring from stainless steel to beryllium copper further when the cryogenic refrigerator employing sealing rings made of beryllium copper is continuously.