There are reports that lightning strikes to bulk transmission lines have caused serious accidents involving electrical apparatus, such as failures of electrical transmission. It is important to analyze the transient characteristics of grounding systems of electrical apparatus in order to understand all phases of such accidents and to apply the results to accident prevention. However, the transient characteristics have so far been difficult to evaluate. Therefore, a method that can easily be used in grounding systems under various conditions for computational analysis of transient grounding resistance (TGR) is highly desirable. We have applied the first such method, which is based on the Finite Difference Time Domain method (FD-TD), to analysis of TGR. The characteristic features of this method are as follows: (i) It can be used to simulate propagation phases of a transient electromagnetic field and enable their visualization. (ii) Because the FD-TD method is a type of computational analysis method in the time domain, the calculated results can be more intuitively and easily understood than those of analysis method in frequency domain. (iii) It can be used to precisely model grounding systems, although there are limitations depending on the computer resources available. Using this method, the TGR at the foot of towers for 500 kV 1,000 kV transmission lines was calculated and compared with the measured results. The relationship between calculated TGR and conductivity of the ground agreed with the measured results. This calculation method for TGR based on the FD-TD method is expected to provide a practical understanding of all phases of accidents and facilitate the rational design of grounding systems. It is also expected to contribute to the prevention of serious accidents involving electrical apparatus.