A sealing structure comprising: a shaft provided with an annular groove on an outer peripheral surface side thereof; a housing having a shaft hole through which the shaft is to be inserted; and a seal ring made from resin which seals an annular gap between the shaft and the housing by closely adhering to each of a sidewall surface of the annular groove on a lower pressure side and an inner peripheral surface of the shaft hole and which has an abutment joint part at one position in a circumferential direction thereof, wherein the seal ring is constituted by a thin part which is thin in a radial direction thereof and provided in a range less than 180° with respect to the circumferential direction, and the abutment joint part is provided in the thin part, characterized in that the seal ring is further constituted by a thick part which is thicker in the radial direction than the thin part by being configured to protrude inward in the radial direction from an inner peripheral surface of the thin part, and the thick part has, at respective ends of both side surfaces on an inner peripheral surface side thereof, a concave part over an entire range thereof, the concave parts form a gap between the ends of the both side surfaces and corner parts at both ends of a groove bottom of the annular groove, wherein the corner parts at the both ends of the groove bottom of the annular groove are constituted by a curved surface connecting the groove bottom of the annular groove and a groove side surface to each other, and both RA > RC and RBX > RC > RB > RD are satisfied, where RA is a curvature radius of the inner peripheral surface of the thin part, RB is a curvature radius of the inner peripheral surface of the thick part, RBX is a curvature radius of a largest region in the radial direction of a portion in which the concave parts are provided, RC is half an outer diameter of a largest region in the radial direction of the corner parts constituted by the curved surface, and RD is half an outer diameter of the groove bottom of the annular groove. a shaft provided with an annular groove on an outer peripheral surface side thereof; a housing having a shaft hole through which the shaft is to be inserted; and a seal ring made from resin which seals an annular gap between the shaft and the housing by closely adhering to each of a sidewall surface of the annular groove on a lower pressure side and an inner peripheral surface of the shaft hole and which has an abutment joint part at one position in a circumferential direction thereof, wherein the seal ring is constituted by a thin part which is thin in a radial direction thereof and provided in a range less than 180° with respect to the circumferential direction, and the abutment joint part is provided in the thin part, characterized in that the seal ring is further constituted by a thick part which is thicker in the radial direction than the thin part by being configured to protrude inward in the radial direction from an inner peripheral surface of the thin part, and the thick part has, at respective ends of both side surfaces on an inner peripheral surface side thereof, a concave part over an entire range thereof, the concave parts form a gap between the ends of the both side surfaces and corner parts at both ends of a groove bottom of the annular groove, wherein the corner parts at the both ends of the groove bottom of the annular groove are constituted by a curved surface connecting the groove bottom of the annular groove and a groove side surface to each other, and both RA > RC and RBX > RC > RB > RD are satisfied, where RA is a curvature radius of the inner peripheral surface of the thin part, RB is a curvature radius of the inner peripheral surface of the thick part, RBX is a curvature radius of a largest region in the radial direction of a portion in which the concave parts are provided, RC is half an outer diameter of a largest region in the radial direction of the corner parts constituted by the curved surface, and RD is half an outer diameter of the groove bottom of the annular groove.