Our previous paper reported the fundamental dynamic characteristics of cracked rotors, crack detection methods by static tests, and a numerical vibration-analysis method of general cracked rotors. This paper describes an example of the diagnosis of a cracked rotor based on former results. A wind-tunnel fan showed a large 2N vibration at 465rpm and was inferred to have a crack from the excitation test, and a shaft crack was detected. The analysis of natural frequencies and responses of the cracked rotor shows the adequateness of the method.