A chain comprising a seal ring for sealing the chain, wherein the seal ring is installed between an outer link plate which is arranged in an outer peripheral side of a connection pin, an inner link plate which is arranged in the outer peripheral side of said connection pin via a bush, and said bush in a state of being compressed in an axial direction, the seal ring comprising: an annular base portion; and an inner peripheral projection which is provided so as to be directed to an inner side in the diametrical direction and has a round surface having a circular arc-shaped cross section in an inward end, wherein said seal ring is adapted to be diagonally twisted by generation of rotation moment on the basis of contact of said inner peripheral projection with said bush when said bush is pressed into an inner peripheral side of said seal ring, and the inner peripheral projection is arranged in an outer peripheral side of an end portion of the bush in place of a gap between the bush and the outer link plate in a state in which the seal ring is diagonally twisted, wherein a first outer peripheral lip which is provided diagonally so as to be directed to an outer side in a diametrical direction and one side in an axial direction from said annular base portion; a second outer peripheral lip which is provided diagonally so as to be directed to the outer side in the diametrical direction and the other side in the axial direction from said annular base portion; a first end surface lip which is provided so as to be directed to one side in the axial direction from said annular base portion; and a second end surface lip which is provided so as to be directed to the other side in the axial direction from said annular base portion; wherein said seal ring, wherein said first outer peripheral lip and the second outer peripheral lip are adapted to come into contact with said outer link plate or the inner link plate in a state in which the first outer peripheral lip and the second outer peripheral lip collapse into an outer side in the diametrical direction, characterized in that the inner peripheral projection is formed as a whole into the approximately circular cross section, by setting a width (d 2) in the axial direction of a neck portion, which is a connection portion to the annular base portion, smaller than a maximum width (d 1) in the axial direction of the inner peripheral projection. an outer link plate which is arranged in an outer peripheral side of a connection pin, an inner link plate which is arranged in the outer peripheral side of said connection pin via a bush, and said bush in a state of being compressed in an axial direction, the seal ring comprising: an annular base portion; and an inner peripheral projection which is provided so as to be directed to an inner side in the diametrical direction and has a round surface having a circular arc-shaped cross section in an inward end, wherein said seal ring is adapted to be diagonally twisted by generation of rotation moment on the basis of contact of said inner peripheral projection with said bush when said bush is pressed into an inner peripheral side of said seal ring, and the inner peripheral projection is arranged in an outer peripheral side of an end portion of the bush in place of a gap between the bush and the outer link plate in a state in which the seal ring is diagonally twisted, wherein a first outer peripheral lip which is provided diagonally so as to be directed to an outer side in a diametrical direction and one side in an axial direction from said annular base portion; a second outer peripheral lip which is provided diagonally so as to be directed to the outer side in the diametrical direction and the other side in the axial direction from said annular base portion; a first end surface lip which is provided so as to be directed to one side in the axial direction from said annular base portion; and a second end surface lip which is provided so as to be directed to the other side in the axial direction from said annular base portion; wherein said seal ring, wherein said first outer peripheral lip and the second outer peripheral lip are adapted to come into contact with said outer link plate or the inner link plate in a state in which the first outer peripheral lip and the second outer peripheral lip collapse into an outer side in the diametrical direction, characterized in that the inner peripheral projection is formed as a whole into the approximately circular cross section, by setting a width (d 2) in the axial direction of a neck portion, which is a connection portion to the annular base portion, smaller than a maximum width (d 1) in the axial direction of the inner peripheral projection.