As the number of delicate semiconductor devices in the electric equipment increases, it is also getting more important to prohibit the surge wave from intruding the equipment. The better surge-wave protection can be achieved when we reduce the earth resistance further in the earthing system. It is practically best, however, to set an appropriate value of earth resistance if we consider the cost and effort required to install the earthing system. In this work, we demonstrated an example of extracting an optimized earth resistance by using a simulator that theoretically predicts the evolution of discharge current of the earthing system. The measure discharge currents of the real earthing system with the optimum earth resistance has confirmed that our method was efficient and useful.