PURPOSE: To reduce the eddy-current loss of a ring-shaped inner tank due to a fluctuating magnetic field and to reduce an AC loss in a superconducting magnet by a method wherein a conductive metal film which makes one round is formed on the whole surface of the inner tank so as to be cut into pieces by an electric discontinuity bands in a plurality of parts and conductive metal film bands are formed on the back of the inner tank so as to correspond to them.

CONSTITUTION: A copper-plated layer 10 as a metal film whose conductivity is good is formed on the surface of an inner tank 4, the copper-plated layer as the film is removed partly, breaks (electric discontinuity zones) 11 in which stainless steel as a preform has been exposed are formed and copper-plated layers 12 are formed on the inner-layer back corresponding to the breaks 11. Thereby, the metal films on the inner-layer back corresponding to the breaks in the metal film on the inner-layer surface suppress a magnetic field creeping from the breaks on the inner-layer surface to be low, the AC loss of a superconducting coil is reduced, and it is possible to suppress that heat is generated.