PURPOSE: To increase stability (heat capacity) without increasing a cross-sectional dimension by covering with Ni or Ni alloys the inner part of a copper material with which a plurality of mono-filament wires are covered.

CONSTITUTION: This is composed of a plurality of mono-filament wires 5 including a plurality of Nb-Ti alloys 3 and a plurality of copper materials 4 with which each of the Nb-Ti alloys 3 is covered and a copper material 1 with which those wire rods 5 is covered and Ni or Ni alloys with which the inner part of the copper material 1 is covered. A coating metal 2 in the inner part is made to have a volume proportion between 5% to show stability effect on the whole of the wires and 30% to make a superconducting magnet larger. And since the specific heat per unit volume of Ni or Ni alloys is about five times as large as that of Cu, even a wire with a copper ratio of not more than 2 way increase stability (heat capacity) without increasing its cross-sectional dimension.