A seal ring that is mounted when in use, in an annular groove provided on an outer circumference of a shaft and seals an annular gap between the shaft and a housing to keep fluid pressure in a region to be sealed that is configured such that the fluid pressure therein changes, said shaft and said housing rotate relative to each other, the seal ring making intimate contact with a low-pressure-side side wall surface of the annular groove and sliding against an inner circumferential surface of a shaft hole of the housing through which the shaft is inserted, wherein, on an outer circumferential surface side of the seal ring, a pair of concave portions extending in a circumferential direction is provided on either side in a width direction, characterized in that a distance between a side surface on one side of a convex portion that is disposed between the pair of concave portions and a side surface on another side of the seal ring, and a distance between a side surface on another side of the convex portion and a side surface on one side of the seal ring are set to be shorter than a distance between an inner circumferential surface of the seal ring and an outer circumferential surface of the convex portion, thereby configuring an effective pressure-receiving area on an inner circumferential surface side of the seal ring, that in use contributes to a force from the fluid pressure that presses against the inner circumferential surface of the shaft hole to become smaller than an effective pressure-receiving area on a side surface side that contributes to a force from the fluid pressure that presses against the low-pressure-side side wall surface of the annular groove, wherein a plurality of ribs connected to the convex portion are provided within the pair of concave portions. the seal ring making intimate contact with a low-pressure-side side wall surface of the annular groove and sliding against an inner circumferential surface of a shaft hole of the housing through which the shaft is inserted, wherein, on an outer circumferential surface side of the seal ring, a pair of concave portions extending in a circumferential direction is provided on either side in a width direction, characterized in that a distance between a side surface on one side of a convex portion that is disposed between the pair of concave portions and a side surface on another side of the seal ring, and a distance between a side surface on another side of the convex portion and a side surface on one side of the seal ring are set to be shorter than a distance between an inner circumferential surface of the seal ring and an outer circumferential surface of the convex portion, thereby configuring an effective pressure-receiving area on an inner circumferential surface side of the seal ring, that in use contributes to a force from the fluid pressure that presses against the inner circumferential surface of the shaft hole to become smaller than an effective pressure-receiving area on a side surface side that contributes to a force from the fluid pressure that presses against the low-pressure-side side wall surface of the annular groove, wherein a plurality of ribs connected to the convex portion are provided within the pair of concave portions.