Nonlinear ultrasonic testing is considered a more promising technique for evaluating closed cracks than conventional ultrasonic testing. However, the mechanism of the generation of nonlinear ultrasonic waves has not been sufficiently explained. We first set up a system to measure the frequency–response characteristics of ultrasonic waves and experimentally investigated the mechanism of second higher-harmonic (HH) wave generation for a fatigue crack. Sweeping the frequencies of incident waves impinging on a fatigue crack introduced to a specimen, we obtained a frequency–response characteristic curve for the crack. From the curve, resonance phenomena resulting from local defect resonance were observed. We then measured the frequency response characteristics of second HH waves using the same system and consequently confirmed that second HH waves resonated when their frequencies corresponded to the eigenfrequencies of the crack. Finally, we theoretically showed that the resonant second HH waves were generated by local defect resonance and nonlinearity.