When a huge tsunami attacks on a composite breakwater, overtopping flow can scour a caisson mound and a seabed and the resultant growth of scour hole may cause sliding or overturning of a caisson. Conventional design methods, which do not suppose this kind of catastrophic failure, are not the reliable tool to predict a structural resistivity against a huge tsunami attack. While, a numerical wave flume to provide time-dependent solution of Navier-Stokes equation can be a key tool to show a rational answer. In this study, a failure of composite breakwater brought by a scouring of seabed is simulated by an accurate particle method. And a measure to prevent the failure is examined by the numerical wave flume.