The conventional ultrasonic inspection is a detection of the ultrasonic wave reflected at the abnormal points in an inspected object. However, the ultrasound passes through an indistinct or minute abnormality. The conventional ultrasonic inspection is unavailable to detect them. In this paper, the authors focus their attention to frequency modulation of the ultrasonic wave, and attempt to detect the minute abnormality by using it. In the proposed method, the frequency modulation is observed as a variation of instantaneous frequency of the ultrasonic wave, and the instantaneous frequency is defined as the derivative of a time-dependent phase angle of the ultrasonic wave. In the case that the indistinct or minute abnormality exists in an object, the authors find that the instantaneous frequency of the ultrasound passing through the abnormality deviates from usual frequency level. Therefore, the abnormality can be detected by the variation of the instantaneous frequency, that is, the frequency modulation. It is difficult for the usual Fourier transform to catch the modulation because of a limitation of a frequency resolution. A feasibility of the proposed method is demonstrated through a simple experiment.