The corrosion of steel structures, such as light and road sign poles, often occurs near the ground due to rainwater. The subsequent collapse of the corresponding steel structure can cause traffic accidents and potentially danger people. Therefore, a highly accurate and easy nondestructive inspection technique is required to detect corrosion near the ground, especially in hidden locations. Recently, we reported a novel inspection method using extremely low-frequency eddy current testing (ELECT) to detect the reduced steel thickness that was caused by corrosion. Wide magnetic-field exposure was achieved underground using a tilted magnetic sensor probe. Further, the fundamental detection performance and signal analysis of the test samples were also reported. In this study, we attempted to improve the detection performance and applied the measurement to actual steel structures as a field test. The ELECT system to detect corrosion near the ground comprised a magnetic sensor probe, a sensor circuit, a multiple-frequency alternative current (AC) current source for the induction coil, a lock-in detector, and a personal computer. The magnetic probe consisted of two anisotropic magnetic resistance sensors, induction coils to generate eddy currents in the steel structures, and the cancellation coil to reduce the directly coupled magnetic field exposure. The magnetic spectrum curve, which can be used to trace the obtained magnetic field vectors (intensity and phase), was acquired by multiple-frequency exposure. The reduction in steel thickness was further estimated based on the changes in magnetic curve. The optimization of certain parameters, such as the exposure coil shape and lift-off between the sensor probe and the steel, led to improvements in the magnetic curve, which enabled sensitive detection; further, 1-mm thinning was successfully detected at a depth of 60 mm. Furthermore, the results obtained by the ELECT system at the ground surface were in good agreement with the results of the thickness measurements that were directly obtained from the structure after digging into the ground.