Conventional eddy current testing (ECT) using a pickup coil is widely employed for detecting cracks. However, it is sometimes difficult to apply conventional ECT to steel structures because of the magnetic noise caused by the non-uniform permeability of steel. To combat this challenge, we developed a small magnetic sensor probe with dual-channel tunnelling magnetic resistance capable of reducing magnetic noise. To apply the developed magnetic sensor probe to a complicated component of a steel structure, such as the welds joining the U-rib and deck plate, signal fluctuations caused by the lift-off variations between the sensor probe and the subject during magnetic sensor probing must be resolved. Therefore, we developed a method to distinguish between the signal changes caused by lift-off variations and cracks. The intensity and phase of the subtracted signal were used to develop traces of differential parameters obtained by scanning the magnetic probe. A visualization technique using only the traces of differential parameters, i.e. the Lissajous curve of X-Y processing, was applied not only to the cracks of the flat steel plate but also to the cracks in welded angles. The developed Lissajous curve emphasized the crack signals, which allowed clear differences between the signal change caused by the lift-off variations and cracks to be observed.