PURPOSE: To reduce blow-by and to abate b oil rise.

CONSTITUTION: An upper sirface 7 of a double-faced keystone ring 6 has a conical surface 7a, a ring horizontal surface 7b at the outside and a chamfering surface 7c at an outer peripheral end, and the lower surface 8 has a conical surface 8a, a ring horizontal surface 8b at the outside. An upper surface 9 of a ring groove 5 of the piston 3 fitted with the ring 6 has a conical surface 9a and a ring horizontal surface 9b at the outside, and the lower surface has a conical surface 10a and a ring horizontal surface 10b. These conical surfaces, where the ring 6 and the ring groove 5 are opposed to each other, are formed in making the same angle with each other. As for the ring 6, when a distance of up to a ring outer peripheral surface 11 at the outside in the radial direction from a boundary between the conical surfaces 7a, 8a and the horizontal surfaces 7b, 8b is set down to L1, and as for the ring groove 5 of the piston 3, when a distance of up to a cylinder inner circumferential surface 12 at the outside in the radial direction from a boundary between the conical surfaces 9a, 10a and the horizontal surface 9b, 10b is set down to L2, they are in relation of L1>L2.