Non-destructive evaluation (NDE) of steel structures, such as bridge and heavy machine, is important issue to ensure their safety and reliability. However, the conventional NDE methods for these structures require long inspection time and high cost because inspections are generally performed manually. Moreover, the measurement in high-place work is also required for the inspection of huge steel structure. Therefore, the measurement system should be compact and an automatically controllable measurement system is desirable. Considering these requirement, NDE using magnetic method is suitable because the measurement system is simple and non-contact measurement are possible . In this study, an automatic scanning system which can move on a wall and ceiling of steel structure was developed. In addition, to demonstrate the developed system for practical measurement, back-side defect caused by corrosions generated in steel structures of cargo crane was measured.