PURPOSE: To prevent increase of heat intrusion even if a plurality of measurement wires are used by varying the size of a measurement wire that connectes a measurement lement to the instrument in accordance with the range of temperature of a location where the measurement wire is positioned.

CONSTITUTION: A plurality of superconducting coil 2 is placed in a vacuum heat insulating vessel 4. Measurement elements 5 of various kinds are attached to the coil 2, and the current for receiving and supplying from this element 5 is through a measurement wire 6. this measurement wire 6 is connected to the point E of an external measurement instrument through a point B that is a very low temperature area and a point C of a precooling area and further a point D that is a normal temperature area. The size of the measurement wire 6 is made relatively large between A and B and very small between B and C in order to prevent heat intrusion. With this arrangement the state of superconductivity can be stabilized and the reliability of the instrument can be improved.