A swage fastening structure comprising: a first plate member; and a second plate member, wherein the first plate member includes a plate main body and a T-shaped swaging piece formed to project from the plate main body in a direction extending along a plane of the plate main body, the second plate member includes a fastening hole through which the swaging piece passes, the swage fastening structure fastens the first and second plate members (10, 20) by swaging the swaging piece in a state where the swaging piece has passed through the fastening hole, the swaging piece includes: a shaft portion projecting from the plate main body in a projecting direction in which the swaging piece projects; and a pair of holding pieces that is formed to be continuous with the shaft portion at a part of the shaft portion away from the plate main body and to project from the shaft portion in both intersecting directions intersecting the projecting direction, and holds the second plate member between the plate main body and thereof, the pair of holding pieces is configured such that an end surface of each holding piece close to the plate main body is inclined in a direction of gradually getting away from the plate main body from one end close to the shaft portion to the other end thereof, and is formed such that a dimension between the plate main body and one end of the end surface close to the shaft portion is smaller than a plate thickness of the second plate member and a dimension between the plate main body and the other end of the end surface is larger than the plate thickness of the second plate member, whereby in a state where the swaging piece has passed through the fastening hole, the shaft portion is configured to be twisted about the shaft portion so that the shaft portion is plastically deformed, thereby the swaging piece is swaged with respect to the fastening hole, and the first plate member and the second plate member are fastened, characterized in that the fastening hole includes: a slit-like holding-piece through hole through which the pair of holding pieces is to pass; and a continuous hole that is formed to be continuous with the holding-piece through hole and allows the shaft portion to rotate inside thereof, an edge of the continuous hole is provided with a pair of sliding contact portions to be in sliding contact with respective end surfaces of the pair of holding pieces when the shaft portion is twisted, the pair of sliding contact portions is formed such that a dimension between the pair of sliding contact portions is slightly larger than a dimension of the shaft portion in the intersecting directions, and the continuous hole is provided with contact portions each being a flat surface to be in contact with surfaces of the holding pieces at a position where the shaft portion has been twisted by a predetermined angle. a first plate member; and a second plate member, wherein the first plate member includes a plate main body and a T-shaped swaging piece formed to project from the plate main body in a direction extending along a plane of the plate main body, the second plate member includes a fastening hole through which the swaging piece passes, the swage fastening structure fastens the first and second plate members (10, 20) by swaging the swaging piece in a state where the swaging piece has passed through the fastening hole, a shaft portion projecting from the plate main body in a projecting direction in which the swaging piece projects; and a pair of holding pieces that is formed to be continuous with the shaft portion at a part of the shaft portion away from the plate main body and to project from the shaft portion in both intersecting directions intersecting the projecting direction, and holds the second plate member between the plate main body and thereof, the pair of holding pieces is configured such that an end surface of each holding piece close to the plate main body is inclined in a direction of gradually getting away from the plate main body from one end close to the shaft portion to the other end thereof, and is formed such that a dimension between the plate main body and one end of the end surface close to the shaft portion is smaller than a plate thickness of the second plate member and a dimension between the plate main body and the other end of the end surface is larger than the plate thickness of the second plate member, whereby in a state where the swaging piece has passed through the fastening hole, the shaft portion is configured to be twisted about the shaft portion so that the shaft portion is plastically deformed, thereby the swaging piece is swaged with respect to the fastening hole, and the first plate member and the second plate member are fastened, characterized in that a slit-like holding-piece through hole through which the pair of holding pieces is to pass; and a continuous hole that is formed to be continuous with the holding-piece through hole and allows the shaft portion to rotate inside thereof, an edge of the continuous hole is provided with a pair of sliding contact portions to be in sliding contact with respective end surfaces of the pair of holding pieces when the shaft portion is twisted, the pair of sliding contact portions is formed such that a dimension between the pair of sliding contact portions is slightly larger than a dimension of the shaft portion in the intersecting directions, and the continuous hole is provided with contact portions each being a flat surface to be in contact with surfaces of the holding pieces at a position where the shaft portion has been twisted by a predetermined angle.