Provided are a run-flat supporting body whose durability is enhanced while a large number of through holes are formed in an annular shell thereof, and a tire/wheel assembly. The run-flat supporting body (1) is configured of the annular shell (2) and elastic rings (3) and (3). The annular shell (2) works as a supporting surface on an outer peripheral side, and bifurcates into two leg portions on an inner peripheral side. In addition, the annular shell (2) is provided with a large number of the through holes (4) in wall surfaces thereof . The elastic rings (3) and (3) are attached respectively to the two leg portions of the annular shell (2). The run-flat supporting body 1 is supported on a rim of a wheel with the elastic rings (3) and (3). According to a configuration of the present invention, peripheries of the respective through holes (4) are thickened, residual stresses are applied to the peripheries of the respective through holes (4), or each through hole (4) is formed into an elliptic shape having the major axis in a circumferential direction.