PURPOSE: To obtain a uniform and sufficient surface pressure acted to the cylinder wall by a piston ring, by shaping the piston ring in a substantially U- shaped form, and disposing a spring at each of arcuate parts of the piston ring in an internal-combustion engine in which the cylinder and the piston are shaped to have an elliptic cross section or a cross section in the form of an elongated circle.

CONSTITUTION: A piston ring 11 is divided symmetrically into two parts, and each of piston ring elements 11A is shaped in a little used form so that a proper surface pressure can be obtained when the piston ring 11 is fitted in a cylinder. At each of arcuate parts 14 located at the middle part of each piston ring element 11A, a spring receiving recess 15 is formed in a ring groove of a piston 12, and an expansion spring 16 is disposed in each of the recesses 15 in the manner that the opposite ends of the spring 16 are located at the part where the arcuate part and the straight part of the piston ring 11 are joined, so that the spring 16 is compressed toward its central part from the opposite ends when the piston ring 11 is fitted in the cylinder. With such an arrangement, it is enabled to obtain a uniform and sufficient surface pressure acted to the cylinder wall by the piston ring.