PURPOSE: To extend the life of a seal ring by providing a buffer mechanism absorbing a shock load applied to the lip-shaped seal ring in a seal device used for an expansion engine of a helium liquefying refrigerator or the like.

CONSTITUTION: A piston 2 is reciprocatively stored in a cylinder 1, and an expansion chamber 4 is formed on the head wall 3 side of the cylinder 1. A circular groove 5 is provided on the outer periphery of the upper end section of the piston 2, and a lip-shaped seal ring 6 is fitted here. An auxiliary seal device 11 is constituted of a circular groove 12 provided on the outer periphery of the piston 2, an auxiliary seal ring 13 formed into a thin cylindrical shape with self-lubricant fluororesin or the like, and an O-ring 14. Accordingly, even if the pressure in the expansion chamber 4 is abruptly increased, the pressure is first applied to the auxiliary seal device 11, the gas pressure leaking from the auxiliary seal device is applied to the lip-shaped seal ring 6, and the life of the seal ring can be extended.