PURPOSE: To attenuate currents quickly without lowering the breakdown strength of a superconducting coil by controlling a chopper so that power discharged from the superconducting coil is consumed by a discharging resistor.

CONSTITUTION: When a first gate control circuit 12 receives a signal from a quenching detector 10 and sets the flowing rate α1 of a chopper 5 at α0 obtained by formula I, the currents IL of a superconducting coil 6 are attenuated by a maximum gradient, and brought to zero in a short time. A chopper 11 for a discharging circuit is normally in an OFF state, and a flowing rate α2 from a second gate control circuit 13 is zero. When the signal is received from the quenching detector 10, the flowing rate α2 is set so that the stored energy of the superconducting coil 6 is consumed by a discharging resistor 8. The power consumption PR of the discharging resistor 8 is acquired by formula II and power PL discharged from the superconducting coil 6 by formula III. When the flowing rate α2 is controlled so that PR=PL holds at that time, all of the stored energy of the superconducting coil 6 is consumed by the discharging resistor 8, and is not discharged to an AC system 1 at all.