I. Introduction
The finite element method (FEM) is widely employed for analyzing electromagnetic problems. Even if FEM is universal, it is not enough because adequacy of the computed results has to be checked in most of the cases, and experimental means used to be employed to check them. As computer technology has been developed, compatible numerical analyses with FEM have taken the place of them. As one of the compatible ones in eddy current analysis, we have the boundary element method (BEM), which has been widely used for analyzing open boundary electromagnetic problems. In the analysis of eddy currents induced in steel, as the permeability of the plate varies with magnetic field intensity, nonlinear analysis is required. As BEM is based on the principle of superposition, it is difficult to solve nonlinear problems. However, it is well known that even BEM can be applied to the nonlinear analysis by regarding nonlinear terms as virtual sources [1]. Several nonlinear eddy current analyses by BEM have been reported [2], [3].