Provided is a seal ring such that the contact stress of a seal lip section is set so as to be maintained in an appropriate state where trouble such as leakage or early abnormal wear does not occur, that durability is improved while maintaining sealability, that furthermore, the installability of a seal outer peripheral section which fits into an annular groove of a casing is improved, and that the contact surface of the seal lip section is stabilized. The seal ring is characterized in that the top of a key section (29) is formed into a chevron shape on the radial outside of the key section (29), with the center (T) as the vertex; that furthermore, the portion between the key section (29) and a heel section (31) comprises step sections (37a, 37b) in order for the axial thickness to be smaller; that the inside diameter of the step section (37b), which is on the back side, is formed to be larger than the inside diameter of the step section (37a), which is on the front side; that moreover, a tip (39) of a lip section (35) protrudes into a chevron shape; and that the center (P) of a spring groove (43) whereinto is fitted an annular spring (41) that presses the lip tip (39) against a liner (15), is provided in such a way as to be offset from the lip tip (39) toward the lip backside within a range equal to 10% of the propeller-shaft-wise length of the lip back surface width.