PURPOSE: To securely prevent a current lead wire from being burned when a magnet is quenched, by providing a valve body constituting a safety valve which opens when the pressure in a superfluide herium tank exceeds a specified level.

CONSTITUTION: In a connection pipe 14, a valve body 24 is formed into a shape of a conical cylinder, which can be closely fitted on the internal surface of the connection pipe 14, and is inserted from the side of a superfluid herium tank 13, with the connection pipe 14 as a valve body. When a superconducting magnet 2 is quenched, a part of the fluid herium in the superfluid herium tank 12 is vaporized due to the joule heat radiation at the part where the conduction is converted into a normal one. When the pressure increases up to the value where the valve body 24 is lifted, the valve body 24 jumps out of the connection pipe 14 so that the fluid herium and low temperature herium gas spout from the superfluid herium tank 12 side to the ordinary fluid herium tank 13 side, with the result that a section of current lead wires 18a, 18b positioned in the connection pipe 14 directly contacts with a coolant and is cooled, thereby securely preventing burning.