The inspection of the hardened depth of surface-hardening steel is important in the maintenance of various mechanical parts, and for other purposes as well. There is a difference in the electromagnetic properties in the layers with and without hardening. Therefore, the evaluation of the hardened depth is possible by detecting the difference in the electromagnetic characteristics. In this paper, the electromagnetic method for measuring the depth of a hardened layer using an alternating magnetic field of low frequency is proposed. The flux density in the surface-hardening steel is estimated by a 3-D nonlinear finite-element method taking account of the hysteresis characteristics of the layers with and without hardening. In addition, the experimental verification is also carried out.