PURPOSE: To liberate a central conductor from influence of outside magnetic field by forming a layer of superconductive substance at the periphery of a conductor having superconductivity through an insulating substance layer, and thereby shutting off the outside magnetic flux by the layer of superconductive substance on the periphery.

CONSTITUTION: An insulative layer 3 is formed by subjecting the periphery of the sheath 2 of a conductor 1 to heat or chemical treatment. A superconductive layer 4 is formed at the periphery of the insulative layer 3 by means of ion evaporation or coating of superconductive substance. This insulative layer 3 is to prevent the current flowing in the conductor 1 from diverging to the superconductive layer 4. In case external magnetic field is applied, the superconductive layer 4 becomes a magnetic shield owing to Meisner effect, and influence of external magnetic field upon the conductor 1 is eliminated. As a result, the superconductive critical value Ic of the conductor 1 will not sink upon receiving influence of the external magnetic field to ensure that operation is carried out stably with high critical current value Ic.