PURPOSE: To improve the stability in superconducting state against the variation of external thermal agitation and a magnetic field, and to prevent the dielectric breakdown between superconducting wires when the insulation coating of the superconducting wire is damaged by a method wherein a good heat conductive material having electric insulating property is provided around an insulated superconducting wire.

CONSTITUTION: An electrically insulative good heat conducting material 11, provided surrounding the outer circumference of each superconducting wire 2, is formed around a superconducting wire 2 which is insulation-coated formal, for example, and the adhesive such as epoxy material and the like is impregnated therein. The generated heat is quickly diffused by the good heat conductor 11 provided on the circumference of the superconducting wire 2. Besides, as the good heat conductor 11 has electric insulating property, no eddy current is allowed to flow even when a change is made in a magnetic field, and as a result, the superconducting wire is hardly changed to a normal conductive state, and the stability of superconductive state is improved. Also, even when an insulating coating is damaged when the superconducting wire 2 is wound and the like, dielectric breakdown can be prevented, because the electric insulating good heat conductor 11 is positioned between the superconducting wires 2.