PURPOSE: To shift a superconducting wire from a superconductor to a nonsuperconductor in a short time by utilizing a resonance circuit of a winding and a capacitor, and forcibly applying a magnetic field of its critical magnetic field or more to the wire, thereby destroying its superconducting phase.

CONSTITUTION: A large current flows to a circuit by a resonance of a winding L and a variable capacitor C simultaneously upon closing of a switch S, the winding L is energized thereby to generate a strong alternating magnetic field in the gap of an annular core 20. When a magnetic field of the critical magnetic field or more of a superconducting wire G is applied to the wire G, the superconducting phase of the wire G is destroyed, the lead is shifted from a superconductor to a nonsuperconductor in a short time, thereby breaking a normal current I flowing to the main line 10.