The big wave suddenly arrived at the beach, Niigata prefecture Japan in May 2014. And three children were carried off to the sea by the wave, though they played on the beach. When the accident occurred, the significant wave heights was not big. We tried to understand the reason why this accident occurred. Firstly, we reproduced the wave condition when the accident occurred. Secondary, we made the survey and obtained the geographic data for the numerical simulation. Finally, we simulated the wave dynamics by the horizontal two dimensional numerical model and the vertical two dimensional numerical model.