This paper describes large-scale parallel full-wave electromagnetic numerical calculations based on the finite element method (FEM) with an iterative domain decomposition method[1][2]. A stationary vector wave equation for the high-frequency electromagnetic field analyses is solved taking an electric field as an unknown function[3]. Although this solver is capable of detailed, fast and efficient FEM of large-scale high-frequency electromagnetic problems using the iterative domain decomposition method (IDDM) and the corresponding parallel distributed processing environment, it still requires a large number of iterative computation trials and computation time[4]. In this study, we found that increasing the size of the subdomain in the IDDM improves the convergence of the accuracy of the iterative method and reduces the computation time. We are also considering replacing the solver with a subdomain solver that can handle larger subdomains.