A seal ring for sealing an annular clearance between two members, which are made rotatable relative to each other. The seal ring is provided with a relief 5 throughout the circumference of a sliding face side to confront the side wall face 72 of an annular groove 71. This relief 5 is set to have a radially smaller width in the vicinity of a separate portion 2 and a radially larger width at a position apart from the separate portion 2, and the portion of the relief 5 to transit from the narrower portion to the wider portion is set to have a gradually changing width.