PURPOSE: To increase the stability of a superconductive state by cooling each saddle-shaped superconducting coil and its connection by circulating cooling medium in a cooling channel.

CONSTITUTION: A difference in temperature which is caused when a saddle- shaped superconducting coil 7 is allowed to conduct generates a difference in density of liquid helium near a connection 10 and a terminal connection 12. Due to this difference in density, the liquid helium 2 flows from the connection 10 and the terminal connection 12 to the saddle-shaped superconducting coil 7 through an inner surface energy refrigerant lead-in hole in a winding installation shaft 4a, as indicated by arrows. The liquid helium 2 further flows to an outer surface of the winding installation shaft through a commuicating hole 8a and then goes back to the connection 10 and the terminal connection 12 through communicating holes 13a, 14a. The liquid helium circulates in this path. Therefore, the saddle-shaped superconducting coil 7, the connection 10 and the terminal connection 12 are cooled. By strengthening the coolingness of the superconducting coil 7 to an increase of conducting current, the stability of a superconducting state can be increased.