PURPOSE: To increase the heat inflow due to radiation, to increase the evaporation amount of refrigerant gas and to prevent field leads from overheating by forming a film having high emissivity on a radiation shield.

CONSTITUTION: A film 17 having a high emissivity is formed on a radiation shield 2 for shielding the heat inflow from an exterior. Part of radiation heat from an ambient temperature damper 1 is arrived through the film 17 formed on the shield 2 at a refrigerant vessel 5 and a coil mounting shaft 15. The size of the film 17 is so set as to obtain the quantity of heat for evaporating the refrigerant in the amount to obtain refrigerant gas necessary for the cold section of field leads 12. The evaporated refrigerant gas from which the quantity of the gas fed to a heat exchanger 4 is removed is gathered in a field lead tube 12.