The prediction of surge phenomena is required for designing effective lightning protection methodologies, and circuit-theory-based simulation techniques, which assume the transverse electromagnetic (TEM) mode, have traditionally been widely employed for lightning surge analysis. Recently, the finite-difference time-domain (FDTD) method, which is one of the full-wave numerical approaches, has become an effective tool for analyzing surge phenomena in three-dimensional structures and grounding structures thanks to the development of several techniques useful for FDTD-based surge simulations. In this study, we review a three-dimensional FDTD-based surge simulation code developed by CRIEPI, its acceleration using the general-purpose computing on graphics processing units (GPGPU) technique, and its application to surge simulations in a microwave relay station.