PURPOSE: To obtain high dielectric strength performance and high mechanical strength by a method wherein a coil is formed by winding untreated compound system superconducting wire material covered with ceramic paper and subjected to a heat treatment at the temperature higher than 500°C.

CONSTITUTION: An insulating covering layer is formed by winding tapes which are obtained by slitting ceramic fiber paper with predetermined widths tightly on a stabilizing material shaped to have a square cross section or a round cross section with a predetermined overlap. The untreated compound system superconducting wire material which is insulated and covered is wound into a coil of a predetermined shape such as saddle shape or pancake shape and then put into a heat treatment furnace and subjected to the heat treatment at 500°C. The heat treatment conditions are determined following the optimum conditions determined by the types of a superconductor such as niobium-3-tin or vanadium-3-gallium. The organic binder in the ceramic fiber paper is decomposed by the heat under the heat treatment at 500W1000°C but the ceramic fibers are not deteriorated by the heat and, as long fibers interlink each other, sufficient mechanical strength can be given to the insulating covering layer.