A cold head is disposed in an insulating container and cooled by a refrigerator. A superconductor is disposed in the insulating container, contacting the cold head, and is cooled to its superconduction transition temperature or lower by heat conduction. A magnetizing coil is disposed outside the insulating container for applying a magnetic field to the superconductor. Control is performed so that a magnetic field determined considering the magnetic field to be captured by the superconductor is applied. A pulsed magnetic field is applied to the superconductor a plurality of times. Each pulsed magnetic field is applied when the temperature of the superconductor is a predetermined temperature or lower. A maximum pulsed magnetic field is applied at least once in an initial or intermediate stage of the repeated application of pulsed magnetic fields. After that, a pulsed magnetic field equal to or less than the maximum pulsed magnetic field is applied. Pulsed magnetic fields are repeatedly applied while the temperature of the superconductor is lowered. A pulsed magnetic field is applied when the temperature T₀ of a central portion of the superconductor is the superconduction transition temperature or lower and the temperature of a peripheral portion is higher than T₀. The temperature of the entire superconductor is brought close to T₀ to apply another pulsed magnetic field. The magnetizing coil faces at least one of two opposite sides of the superconductor to apply pulsed magnetic fields to the superconductor in its magnetization direction.