A tire/wheel assembly abnormality determining apparatus and a method thereof determines whether there is an abnormality in a tire/wheel assembly in a vehicle provided with the tire/wheel assembly constituted by fixing a tire, that is filled with compressed air, to a wheel. The apparatus includes a tire pressure sensor which directly detects an air pressure in the tire as a tire pressure and which outputs a tire pressure signal indicative of the detected tire pressure; and a determining device which determines whether there is a deformation state abnormality, that is an abnormality regarding a deformation state of the tire, in the tire/wheel assembly, based on frequency characteristics of the output tire pressure signal. Therefore, it is possible to accurately determine whether there is an abnormality in the tire/wheel assembly in the vehicle, regardless of fluctuation of a wheel speed.