Pulse-echo method is a conventional ultrasonic inspection. This method detects a crack as the reflection of ultrasonic pulse at the crack. However, this method is ineffective to detect a closed crack which shows an indistinct reflection. In this study, the authors propose use of instantaneous frequency of the ultrasonic pulse in order to detect the closed crack. The instantaneous frequency can distinguish the slight difference in a frequency of the ultrasonic pulse passing through the crack because the instantaneous frequency expresses the frequency variation in detail. It is difficult for the usual Fourier transform to catch a small change in ultrasonic frequency because of wide spread property and indistinct peak. Feasibility of the proposed method is demonstrated through simple experiments.