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An electromagnetic system was developed to detect the covering depth of steel reinforcing bar (rebar). In this system, a circular coil with the diameter of about 10 cm was used to produce the excitation magnetic field. The frequency of the AC current flow in the excitation coil was 4.2 kHz and the current amplitude was about 20 mA. A coil wrapped around a 1 cm ferrite core was used to detect the induced magnetic field by steel rebar. From the signal amplitude after demodulation, the depth information of the steel rebar was obtained. To detect deeper steel rebar, the big background signal produced by the excitation coil must be compensated. To compensate it, another coil also wrapped around the ferrite core was used. By optimizing the system, the maximum detection depth was about 220 mm. We also measured the resolution of depth measurement. It was about 0.3 mm when the depth was 50 mm, and 0.6 mm when the depth was 80 mm.