PURPOSE: To obtain an outer tank having such structure as damping 6th higher harmonics effectively without increasing the weight of cryostat by making thick the outer tank on the side opposing a ground coil as compared with other sides.

CONSTITUTION: An outer tank 1 is made thick on the side opposing ground coils 5, 6. Consequently, loss due to 6th higher harmonics produced in an inner tank 3 can be suppressed. Furthermore, loss due to a heat shield 2 is also suppressed. 6th higher harmonics in the ground coil can be damped effectively by increasing the thickness of the outer tank and increase of weight can be restricted by making thick the outer tank only on the side opposing the ground coil. Furthermore, quenching of superconducting coil and abnormal evaporation of liquefied helium can also be suppressed.