PURPOSE: To maintain sealability between a displacer and a cylinder by preventing the shrinkage of a seal ring at a cryogenic temperature.

CONSTITUTION: An endless seal ring 6 of a substantially sectional U shape having an inner lip 61 and an outer lip 62 for energizing in the separating direction by an inner energizing unit 8 is mounted on the outer peripheries of displacers 31, 32. A thin plate 9 which is harder than the material of the ring 6 and low shrinkage at a low temperature is interposed at the energizing surface between the ring 6 and the unit 8 to prevent the shrinkage of the ring 6 by the plate 9.