PURPOSE: To make a refrigerant flow smoothly on each insulator junction face by forming a first channel in a junction face of an insulator and a second channel along the longitudinal direction of a superconducting wire.

CONSTITUTION: Liquid helium flows, as indicated by arrows, from a hole 27 of an upper insulator 19 to longitudinal channels 41, 42 on the side face of the upper insulator 19 through a horizontal channel 28, and then flows into channels 29, 30 of side insulators 21, 22 through channels 45, 47 formed in the longitudinal direction of coils of the side insulators 21, 22. After that, the liquid helium flows from the channels 29, 30 to a horizontal channel 31 of a lower insulator 20, passing through channels 46, 48 formed in the longitudinal direction of the coils at the other end of the side insulators 21, 22 and a longitudinal channels 43, 44 on the side face of the lower insulator 20. By this, the refrigerant flows smoothly on each insulator junction face surrounding the coil 3 and thereby a very stable conductive state can be kept.