The leakage of the coolant through a sealing member at ultra-low temperatures is minimized. The sealing member (32) is made of an unsaturated polyester PTFE whose filling ratio of unsaturated polyester is 20 % - 35 %. The end portion (35) of the outer circumferential wall (32a) has a plurality of slits (36) that extend in the axial direction and pierce through the wall in the radial direction. The synergistic effect of the use of a material with a small thermal expansion coefficient and the reduction in overall shrinkage of the outer circumferential wall (32a) in the radial direction resulting from its shape enhances the sealing performance and minimizes the leakage at ultra-low temperatures, thus preventing degradation of the refrigerating capability.
