PURPOSE: To improve reliability and cooling characteristics by a method wherein a terminal voltage potential resistor connected to the opposite ends of a superconductive electromagnet is provided in an insulated container and output voltage terminals at a low-voltage side are led to a room temperature side to compose a device.

CONSTITUTION: In a device consisting of a superconductive electromagnet 1 wound a superconductive wire, insulated container 8 isolated from the surrounding room temperature section, power source 4 for the electromagnet 1, and protective resistor 9, a terminal voltage potential resistor 21 is connected to the opposite terminals of the electromagnet 1, and output voltage terminals 22, 23, 24 at a low voltage side are taken out of a hermetic seal section 10 and connected to a transfer detector 5. In this case, the resistance 21 value is sufficiently higher, resistance R21, than the resistance 9 value and the value of the potential resistance R22 across the terminals 22, 23 is equal to that of the potential resistance R24 across the terminals 23, 24, and the value of resistors, R22, R23 is 1/100 of R21 value, for example, and the terminal 23 is grounded. This permits remarkably low transfer detection voltage as compared with the terminal voltage at the electromagnet 1. Therefore, a small-sized hermetic seal section is available and short-circuit accident can be prevented.