PURPOSE: To improve the stability of a permanent current mode by providing a by-pass circuit having a second permanent current switch in parallel with a permanent current switch, and making the resistance value of the by-pass circuit higher than that of the permanent current switch.

CONSTITUTION: An emergency loop is formed by providing a by-pass circuit composed of a permanent current switch 3 and a series circuit of a microresistor 6 and a permanent current switch 7 connected in parallel to the permanent current switch 3, or a by-pass circuit 8 having a semiconductor switch or contactor as a switch connected in parallel to the permanent current switch 3. And permanent current is caused to flow in this emergency loop, when the permanent current switch 7 is quenched. By adopting constitution like this, the permanent current switch 7 is cooled with liquid helium while the permanent current is flowing in the emergency loop, and returns to a superconductive state by itself. Consequently, it becomes possible to keep the superconducting magnet 2 at a permanent current mode always stably, and increase the reliability of the superconducting magnet.