PURPOSE: To prevent reduction of output and the piston beating noise accompanied by oscillation of the piston by forming in ordinary times a little gap between a piston ring and an expander ring, in the apparatus having both rings in the ring groove on the outer peripheral surface of the piston.

CONSTITUTION: The outer peripheral piston ring 4 which contact with the inner peripheral surface of a cylinder 1 and the expander ring 5 which is bent in wave form are installed in the ring groove 3 formed on the outer peripheral surface of the piston 2 which is fitted in the cylinder 1. A small gap 8 is formed in ordinary times between the expander ring 5 and thepiston ring 4. Therefore, the piston ring 4 is brought into contact with the inner peripheral surface of the cylinder 1 by the self springy force in ordinary times and kept at a relatively low surface pressure. When oscillation is generated on the piston 2, and the ring 4 is pushed from outside, the ring 4 is supported from inside by the ring 5 described the above, and oscillation is resticted and generation of beating noise is prevented.