PURPOSE: To increase the hardness of the silver sheath and to improve the degree of compaction of superconducting fine powder by compacting a silver sheath by a material obtd. by mixing a small amt. of Mo fine powder into pure silver fine powder.

CONSTITUTION: A small amt. of Mo fine powder is mixed into pure silver fine powder into a dispersion strengthened pipe, and at the time of working the pipe, heat treatment in accordance with the reduction of the cross section is executed to grow the crystals while the Mo fine powder in the silver matter is internally oxidized. In this way, the fluidity of the plasticity in the pure silver itself is prevented, so that the hardness of the silver sheath is macroscopically strengthened. Furthermore, without giving trouble to the absorption of oxygen as the property of the pure silver itself, working can easily be executed and its hardness is sufficiently improved corresponding to the densification of the superconducting powder packed into the silver sheath. By using the above silver sheath, a superconducting wire or tape increased in current density can be obtd.