A worm reducer for an electric power steering system comprising: a worm shaft that has a first end portion and a second end portion in an axial direction of the worm shaft, the second end portion coupled to an electric motor; a worm wheel that meshes with the worm shaft; a bearing that supports the first end portion of the worm shaft; a housing including an inner surface having a portion that defines a first end housing portion that houses the first end portion of the worm shaft and the bearing, the portion of the inner surface provided with a recess having a pair of inner wall surfaces (82, 83) that face each other in a circumferential direction of the bearing; an urging member that urges the first end portion of the worm shaft toward the worm wheel via the bearing; and an elastic member including an arc-shaped buffer portion that is interposed between the portion of the inner surface of the housing and an outer peripheral surface of the bearing and that opens toward the worm wheel, and a rotation regulation portion that extends from the buffer portion and is housed in the recess and that regulates the buffer portion from rotating in the circumferential direction by engaging with the inner wall surfaces (82, 83), wherein the rotation regulation portion includes a hollow portion, characterized in that the hollow portion is configured to suppress load transmission from the rotation regulation portion to the buffer portion, the rotation regulation portion includes a pair of engagement portions that are spaced away from each other in the circumferential direction of the bearing and each engage with a corresponding one of the pair of inner wall surfaces (82, 83) of the recess of the housing, and the hollow portion is interposed between the engagement portions, and is formed of a through hole or a non-through hole that has a depth in a direction parallel to the axial direction of the worm shaft. a worm shaft that has a first end portion and a second end portion in an axial direction of the worm shaft, the second end portion coupled to an electric motor; a worm wheel that meshes with the worm shaft; a bearing that supports the first end portion of the worm shaft; a housing including an inner surface having a portion that defines a first end housing portion that houses the first end portion of the worm shaft and the bearing, the portion of the inner surface provided with a recess having a pair of inner wall surfaces (82, 83) that face each other in a circumferential direction of the bearing; an urging member that urges the first end portion of the worm shaft toward the worm wheel via the bearing; and an elastic member including an arc-shaped buffer portion that is interposed between the portion of the inner surface of the housing and an outer peripheral surface of the bearing and that opens toward the worm wheel, and a rotation regulation portion that extends from the buffer portion and is housed in the recess and that regulates the buffer portion from rotating in the circumferential direction by engaging with the inner wall surfaces (82, 83), wherein the rotation regulation portion includes a hollow portion, characterized in that the hollow portion is configured to suppress load transmission from the rotation regulation portion to the buffer portion, the rotation regulation portion includes a pair of engagement portions that are spaced away from each other in the circumferential direction of the bearing and each engage with a corresponding one of the pair of inner wall surfaces (82, 83) of the recess of the housing, and the hollow portion is interposed between the engagement portions, and is formed of a through hole or a non-through hole that has a depth in a direction parallel to the axial direction of the worm shaft.