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 1.2m, and the wave period was 7.9 seconds. The beach characteristic topography has the cusp topography and steep slope. 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 to understand the geographic feature of the beach. After the survey, we obtained the geographic data for the numerical simulation from the aerial photograph which were taken by UAV (Unmanned aerial vehicle). Finally, we comprehended the wave dynamics on the beach by the numerical simulation. We simulated the wave dynamics by the horizontal two dimensional numerical model and the vertical two dimensional numerical model.