To prevent concentration of stress of a top ring by arranging a plurality of groove parts along the inner side surface of the top ring fit to the piston circumferential surface of an internal combustion engine, and forming the depth of those groove parts deeper toward the vicinity of the fitting port part thereof.

A total of four groove parts consists of two groove parts 6 and two groove parts 6a are arranged on the inner side of a top ring 3 along the axial line of a piston. The depth of the recessed shaped groove parts 6, and the groove parts 6a are formed deeper toward the vicinity of the fitting port part 7 thereof. Stress concentration by fluttering of the different phase of the fitting port part 7 and stress concentration by twist of the top ring 3, which are troublesome in the case of enlargement of side clearance, are dispersed in a longitudinal direction of the top ring 3 by those recessed groove parts 6 and the groove parts 6a, and they are absorbed. A stress value applied on the top ring 3 is made in a constant level by deepening it on a position where stress is in a low level.