The conventional ultrasonic inspection is a detection of the reflected ultrasonic pulse from the abnormal point in an inspected object. However, an indistinct abnormality does not reflect ultrasound. The conventional ultrasonic inspection is in unavailable to detect it. In this study, the authors propose to use an instantaneous frequency of the ultrasonic pulse. The instantaneous frequency can show the significant difference in a frequency of the ultrasonic pulse passing through the indistinct abnormality because the instantaneous frequency can express the frequency variation in detail. It is difficult for the usual Fourier transform to catch a very small change in ultrasonic frequency because of limitation of a frequency resolution. A feasibility of the proposed method is demonstrated through a simple experiment.