An object of the invention is to obtain a similar side sealing effect to what would be obtained when a means for cutting a ring corner angled portion for improvement of side sealing is used for a compression ring (1) having a special abutment comprising a projecting portion (2) provided in an upper corner angled portion of one of abutment end portions and a recessed receiving portion (3) provided in the other abutment end portion for receiving the projecting portion without using the same means and to enlarge the cross-sectional area of the projecting portion (2). To this end, grooves (14, 15) are formed in an inner circumferential surface (16) of a compression ring (11) in such a manner that centers of those grooves are positioned above a transverse center of the ring. Since the necessity of inner cutting is obviated, it is possible to enlarge the cross-sectional area of a projecting portion (12).