PURPOSE: To enable a high temp. superconductor to be maintained in a superconducting state even when the superconductor undergoes a high thermal load, by adding liq. carbon tetrafluoride to a cryogenic cooling medium used for a high temp. superconductor and composed of liq. nitrogen.

CONSTITUTION: Liq. nitrogen is mixed with liq. carbon tetrafluoride to produce a cryogenic cooling medium. A pref. mixing ratio is such that the vol. ratio of liq. nitrogen to liq. carbon tetrafluoride is about 9:1. In the use of this cooling medium, a high temp. superconductor 1 is placed in a low temp. vessel 2, a mixed cooling medium 12 prepared by mixing liq. nitrogen 6 with liq. carbon tetrafluoride 9 is introduced into the low temp. vessel 2 so as to cool the high temp. superconductor 1 to such an extent that a superconducting state is attained, and an electric current is passed from a power source 4 to form a superconducting magnet. In this case, since the mixed cooling medium 12 maintains a nucleate boiling state even when it undergoes a thermal load of, e.g., 20W/cm2 or above, it is possible to maintain the superconductor 1 in a superconducting state.