This study purpose is an improvement of convergence properties of an interface problem in the timeharmonic eddy current analysis based on the iterative domain decomposition method. A governing equation based on the A-φ method for the time-harmonic eddy current analysis is solved taking the magnetic vector potential A and the electric scalar potential φ as unknown functions. However, when a numerical model become huge, the convergence properties of the interface problem get worse. To overcome this difficulty, we introduce a pseudo-quadruple precision into iterative solvers in the interface problem and the subdomain problem. Convergence properties in the interface problem are improved by applying the pseudo-quadruple precision to both solvers for the interface problem and the subdomain problem.