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Fatigue cracks occurred in the welds of steel structure are easily propagated. Therefore, it is important to detect the fatigue cracks at an early stage. However, most of the steel structures are protected by surface coating, which requires removing and restoring the coating for the inspection. This makes the inspection time-consuming. Magnetic methods such as eddy current testing (ECT) and magnetic flux leakage (MFL) can be used without removal of surface coating. However, these magnetic methods are difficult to apply to the shape of the welds and affected by nonuniform magnetic permeability. In this study, we developed a sensor probe adapted to the shape of the welded joint using the unsaturated ac MFL (USAC-MFL), which is less affected by the nonuniform magnetic permeability. To evaluate the performance of the developed sensor probe, the welded joint samples with the surface coating and shallow cracks of which depth ranges from 1 to 5 mm were measured. The measurement results showed a strong correlation between the intensity of the MFL and the crack depth. The detection performance was evaluated using the confusion matrix and the receiver-operating characteristic (ROC) curve. The area under curves (AUCs) of the ROC curves were around 0.9. In order to maximize the true positive rate (TPR) and minimize the false negative rate, the threshold of the intensity to identify a crack was obtained using the ROC curve. As the results of evaluating the detection performance using the obtained threshold, the four metrics (accuracy, precision, recall, and specification) were all above 0.8. [ABSTRACT FROM AUTHOR]