Fatigue cracks of more than several hundred millimeters in length are found and repaired upon maintenance, which is now executed by visual check. These fatigue cracks are detected in and through a secondary phenomenon, as local corrosion, water leakage, and abnormal vibration. The presence of fatigue cracks, however, cannot be directly detected. This paper proposes a method to detect fatigue cracks from the early phase of infinitesimal fatigue cracks, in which present visual maintenance judges that there is no fatigue damage. A crack size is estimated according to results of crack detection and by FEA. The judgment of fracture possibility and the method to estimate residual strength and remaining fatigue life are considered in accordance with the crack size. The results obtained from this study are as follows: 1) Lowering rate of strain, calculated as the ratio of strain after crack initiation to strain at the time of no damage, is proposed as a monitoring parameter. 2) Monitoring lowering rate of strain from the time of no damage allows a fatigue crack in and after the phase of the infinitesimal crack to be detected. 3) In cases of the changes in cyclic loading, cracks can be detected by lowering rate of strain concentration.