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This paper presents a low frequency eddy current method to estimate internal deep defects in ferromagnetic material. A magnetic system for an exciting field is designed to generate sufficient coil flux to enhance sensitivity and to make certain of signal linearity. The existence and shape of a defect can be recognized by observing the difference in variation of the equivalent impedance. Magnetic systems with various interior deep defects are numerically analyzed by finite element method and their data is compared with those of experimental systems. The measured data of the impedance variation are distinguishable enough to be used to estimate the existence and shape of defects in the steel structure.