PURPOSE: To lengthen the interval of liquid helium injection, and reduce the manhour for re-injection, by winding a material member excellent in thermal conductivity like aluminum together with superconductor wire.

CONSTITUTION: A pure aluminum sheet 8 is wound together with superconductor wire 9. Since the thermal conductivity of the sheet 8 is excellent, the superconductor wire 9 is cooled all over the periphery even when the surface of the liquid helium 4 descends on account of evaporation. As the result, superconducting state can be maintained when the quantity of the liquid helium decreases to some extent. In this manner, the pure aluminum sheet excellent in thermal conductivity is wound together with the superconductor wire, thereby forming a superconducting coil. Hence the superconducting state can be maintained, when the quantity of the liquid helium becomes less than the lowest level which maintains the superconducting state, so that the injection interval of liquid helium is lengthed, and a superconducting magnet can be operated for a long time.