A lot of tubes are used in heat exchanger of thermal power plants and nuclear plants. These tubes are stored as a bundle with support steel plate (baffle), and defects or corrosion generate on the outer side of the steel tubes. It is necessary to inspect outer side defect on these tubes from the inside of the tube. In this paper, problems in applying the alternating flux leakage testing method to the inspection of the outer side defect on steel tube with the baffle are examined using 3-D non-linear FEM.