PURPOSE: To provide a method of realizing permanent current mode operation with reduced operating cost of a superconductor coil for magnetism separation.

CONSTITUTION: A combination of a superconductor coil 10 for the permanent current mode operation and two element coils 12 and 13 of magnetic erasing coils arranged inside the superconductor coil 10 are arranged around magnetic filters 15 and 13. The two elementary coils 12 and 13 of the magnetic erasing coils are wound in the opposite directions to each other and generate mutually- opposite magnetic fields in the region of the magnetic filter 15 and 17 so that the structure does not change the amount of magnetic flux that couples with the superconducting coil 10. Regular operation of the magnetism separation device generates a magnetic field during the permanent current mode operation of the superconducting coil 10 only, the magnetic erasing coil is excited during the cleaning of a magnetic filter and by flowing the exciting current in the reverse direction, the magnetic field around the magnetic filter 15 and 17 are turned to zero in turn and the magnetic filter is cleaned.