PURPOSE: To enable a removable current lead to be mounted in a safe and reliable manner, by providing a gate-type valve at the top end of a connecting tube connecting an inner chamber to a vacuum chamber so that the gate-type valve is opened during mounting of the removable current lead.

CONSTITUTION: In order excite a superconducting coil 1, a blind cap generally attached to the top of a gate-type valve 17 is removed in the first place and a removable current lead 9a is fixed to the gate-type valve 17. A gate 21 is opened by a lever 22 of the valve 17 and a removable current lead 9a is inserted therethrough to be connected to a connector 11 of a low-pressure section. Subsequently, an exciting power supply 23 is connected to excite the coil. According to such construction in which the valve 17 is provided at the top of a connecting tube 7 connecting an inner chamber 2 to a vacuum chamber 3 and the removable current lead 9a is provided removably, a very safe and reliable superconductive magnet which is prevented from spounting out of evaporated gas entrance of air can be obtained.