PURPOSE: To enable a leaking magnetic field to be extremely suppressed with superior reproducibility and productivity, by enabling superconductive coils and their shield to be cooled to a superconductive state.

CONSTITUTION: Superconductive coils 1...1 and a shield 2 are housed to be cooled inside a cryostat. Circular superconductive coils 1...1 are housed inside a coolant vessel 4 arranged inside a cryostat vacuum vessel 3, and immersed in a coolant such as liquid helium. The shield 2 of superconductive material formed in a cage shape is wound around the peripheral surface of the coolant vessel 4. Therefore, the shield 2 is cooled through the wall surface of the coolant vessel 4 to the temperature near the inside coolant temperature, so that it is cooled to the superconductive temperature together with the superconductive coils 1...1, with a superconductive phase being maintained.