PURPOSE: To hold a substance to be cooled, at a very low temperature extending over many hours, by storing a skid and a pickup coil in liquid gas of a double structure vessel, and also making the inside gas pressure higher than the outside gas pressure.

CONSTITUTION: A pickup coil 2 and a skid 3 are stored in the vessel 6, and are cooled by liquid gas 4'. Since this vessel 6 is provided in the low temperature liquid tank 1 which contains liquid gas 4 whose pressure is lower than that of liquid gas 4', the pickup coil 2 and the skid 3 are surrounded and cooled by the double liquid gas 4, 4'. Also, the vessel 6 is formed by a store chamber 6' for storing the skid 3, etc., and a long and slender vertical chamber 6' on the upper part, and also liquid gas 4' is stored up to the upper position of this vertical chamber 6', therefore even if the low temperature liquid tank 1 inclines greatly or shakes, the pickup coil 2 and the skid 3 are not exposed to gaseous phase. According to such a constitution, the skid, etc. can be held at a very low temperature state, extending over many hours.