PURPOSE: To prevent operated gas from leaking, reduce friction force and prevent a lubricant from scattering, by mounting two or more seal rings and sealing a part between these seal rings with the lubricant.

CONSTITUTION: Seal rings 7 and 7' having an unsymmetrical shaped-section are arranged oppositely facing on a piston 5, and a part between these two seal rings is filled with a lubricant. The seal ring 7' in the side of high pressure is provided with a function preventing the lubricant from infiltrating inside a cylinder. The seal ring 7 in the side of low pressure is provided with a function sealing expansive gas in an engine. The seal rings 7, 7' are arranged in a symmetrical shape interposing the part filled with the lubricant.