The evaluation of surface-hardened depth (D) in induction-hardened steel is important in the guarantee of quality of the steel. If the size of the surface-hardened steel is large, a nondestructive evaluation method is required to inspect the D since destructive testing using, for example, a Vickers hardness tester, becomes difficult. The electromagnetic properties of the hardened layer are different from those of the nonhardened layer in surface-hardened steel. Therefore, it is possible to estimate the D of surface-hardened steel by measuring the difference in these electromagnetic characteristics using an electromagnetic sensor. On the other hand, the detection signal in the electromagnetic sensor is also affected by the distance (lift-off: Lo) between the surface of the hardened steel and the sensor. In this paper, a method for simultaneously measuring the D in a hardened steel plate and Lo is proposed. The usefulness of simultaneous measurement using the proposed inspection method is evaluated by a 3D nonlinear finite element method taking account of the initial B–H (magnetic flux density: B, magnetic field strength: H) curve and the conductivity of the layers with and without hardening in the surface-hardened steel. In addition, an experimental verification is also carried out.