A seal ring configured to be attached to an annular groove provided on an outer periphery of a shaft and seal an annular gap between the shaft and a housing rotating relative to each other to maintain fluid pressure in a sealed region in which the fluid pressure is configured to change, the seal ring is configured to be in close contact with a lateral-wall surface on a lower pressure side (L) of the annular groove and slide against an inner peripheral surface of a shaft hole in the housing into which the shaft is to be inserted, wherein the seal ring has, on an outer peripheral surface side thereof, a pair of recessed parts extending in a circumferential direction on both sides in a width direction thereof to form a projection part between the pair of recessed parts , and has a plurality of ribs connected to the projection part and extending to lateral surfaces of the seal ring at intervals in the circumferential direction inside the pair of recessed parts , and a plurality of ribs arranged on a sealed region side (H) among the plurality of ribs have specific lateral-wall surfaces on an upstream side in a relative rotation direction of the housing with respect to the shaft, each of the specific lateral-wall surfaces having inclined surfaces inclined from the upstream side to a downstream side in the relative rotation direction of the housing from the projection part toward the lateral surfaces of the seal ring , characterized in that each of the plurality of ribs gradually narrows toward the lateral surfaces of the seal ring from the projection part such that both lateral-wall surfaces in the circumferential direction of the ribs provided on both sides of the projection part are constituted by inclined surfaces, each of which are inclined with respect to the circumferential direction and the specific lateral-wall surfaces exist regardless of the relative rotation directions of the shaft and the housing and regardless of which one of the both of lateral surface sides of the seal ring is a sealed region. the seal ring is configured to be in close contact with a lateral-wall surface on a lower pressure side (L) of the annular groove and slide against an inner peripheral surface of a shaft hole in the housing into which the shaft is to be inserted, wherein the seal ring has, on an outer peripheral surface side thereof, a pair of recessed parts extending in a circumferential direction on both sides in a width direction thereof to form a projection part between the pair of recessed parts , and has a plurality of ribs connected to the projection part and extending to lateral surfaces of the seal ring at intervals in the circumferential direction inside the pair of recessed parts , and a plurality of ribs arranged on a sealed region side (H) among the plurality of ribs have specific lateral-wall surfaces on an upstream side in a relative rotation direction of the housing with respect to the shaft, each of the specific lateral-wall surfaces having inclined surfaces inclined from the upstream side to a downstream side in the relative rotation direction of the housing from the projection part toward the lateral surfaces of the seal ring , characterized in that each of the plurality of ribs gradually narrows toward the lateral surfaces of the seal ring from the projection part such that both lateral-wall surfaces in the circumferential direction of the ribs provided on both sides of the projection part are constituted by inclined surfaces, each of which are inclined with respect to the circumferential direction and the specific lateral-wall surfaces exist regardless of the relative rotation directions of the shaft and the housing and regardless of which one of the both of lateral surface sides of the seal ring is a sealed region. <EOS>