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Steel tubes are used in the heat exchanger in the petrochemical plant or the steam generator in the fast breeder reactor. Since these steel tubes in the heat exchanger or steam generator are stored as a bundle by the support steel plate (baffle), it is necessary to inspect the existence of defects from the inside of each tube. In these steel tubes, the defects are generated between the steel tube and the baffle at the outer side of steel tube. In this paper, problems in applying the alternating flux leakage testing method for the inspection of the outer side defect on steel tube with the baffle are examined using 3-D nonlinear FEM. Moreover, the differential type search coils that detect the leakage flux from the outer side defect on steel tube are proposed. The optimal dimension of the differential search coils is determined using the Rosenbrock's method or the evolution strategy.