PURPOSE: To interrupt a superconducting unit, where the current of main circuit varies with time, positively by detecting the current flowing through the main circuit and controlling the charging voltage of a commutation capacitor depending on the current thus detected.

CONSTITUTION: A power supply 10 for charging a capacitor is provided with a circuit 12 for controlling the charging voltage of a commutation capacitor 7 based on the output from a detector 11 for detecting the current flowing through the main circuit. Charging voltage of the commutation capacitor 7 is controlled depending on the current flowing through the main circuit in order to optimize the commutation current at the time of interruption of the charging power supply 10. This constitution allows superposition of an optimal commutation current depending on the current flowing through the main circuit thus interrupting the main circuit positively at the time of abnormality and protecting a superconducting coil 1.