In a pressurized superfluid helium cryostat, a first flow rate control valve and a preliminary cooler are installed between a liquid helium inlet at a bottom portion of a 4.2K liquid helium bath and a J-T valve which is a second flow rate control valve. The preliminary cooler is cooled by depressurization by a vacuum pump. The liquid taken in from the 4.2K liquid helium bath is cooled to a temperature of the preliminary cooler by J-T expansion in the first flow rate control valve, and stored in the preliminary cooler. The liquid helium in the preliminary cooler can be cooled below 2.2K, and further subjected to J-T expansion in the J-T valve. By installation of the preliminary cooler, the efficiency of the superfluid cooler can be increased as compared to a conventional type.
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