Methods for reduction in communication time during parallel computation of a particle method for flow simulation of seaside areas are presented. Since the analysis region was divided for parallel computation using a PC cluster, communications among processors were necessary. Two techniques were tested. The first was renumbering, which reduced the amount of communication. The second was the use of a communication list including the maximum and minimum particle numbers. These techniques improved communication speed. Performance during large-scale simulation was examined by computing tsunami wave run-up on a seaside area using 6,300,000 particles. A calculation time of 237 hr was used to generate real time data for 60s.