PURPOSE: To form a uniform magnetic field, in which there is no leakage magnetic field, and to improve the energy efficiency of a magnet by arranging a side plate composed of an oxide superconductor around the gap of a core.

CONSTITUTION: A side plate 5 consisting of an oxide superconductor is disposed around the gap 3 section of a core 1. The line of magnetic force 4 does not leak to the outside by the superconductive Meissner effect of the side plate 5 made up of the oxide superconductor, and the line of magnetic force 4 is confined into the gap 3. Accordingly, the line of magnetic force 4 shapes a uniform magnetic field in the gap 3 while it can generate a stronger magnetic field when the magnetomotive force of a coil is the same, thus improving the energy efficiency of a magnet.