A defect on the opposite-side of a steel plate is detected by an electromagnetic inspection method using a large static magnetic field and a small alternating magnetic field. However, in this inspection method, it is difficult for the inspection sensor to move because strong magnetic attraction is generated between the steel plate and the inspection sensor. To improve the practicality of this inspection method, it is necessary to weaken the magnetic attraction and improve the detection sensitivity. In this paper, an inspection method for opposite-side defects in a steel plate that uses an alternating magnetic field with high and low excitation frequencies is proposed. It is shown that the detection sensitivity of an opposite-side defect in a steel plate using the alternating magnetic field with the two excitation frequencies is higher than that using a large static magnetic field and a minute alternating magnetic field. The phenomenon underlying this technique is elucidated by analysis using 3D nonlinear finite-element method taking account of the hysteresis (minor loop) and eddy current.