I. Introduction
We employ usually two contrastive approaches to analyze numerically the electromagnetic fields. One is the finite element method (FEM) and the other is the boundary element method (BEM). The permeability of steel is usually so large that the skin depth is so small even at a low frequency such as 50 Hz. The analysis by FEM is rather cumbersome in the case of materials such as steel due to the requirement of the volume meshes with flat shape, while BEM is able to solve the boundary integral equations (BIEs) by employing surface meshes in linear problems. From this viewpoint, BEM has a significant advantage over FEM.