The numerical simulator, STOC, which consists of 1) 3D models of STOC- IC and STOC-VF, 2)multilevel model named STOC-ML and 3) connection model, has been developed to provide seawater motion from offshore to coastal zones due to tsunamis or storm surges including inundation onto a complex beach. By adopting the 3D model locally, we can economically and accurately evaluate complicated flow around coastal structures or over various topographies, which are smoothly expressed with a porous model introduced into the continuity and Reynolds equations.Water surface displacement is determined by the vertically integrated equation of continuity in both STOC-IC and STOC-ML, while by the 3D-VOF method in STOC-VF. In comparison with experimental, theoretical and numerical data, the calculation results show accuracy of STOC in thevarious test cases. In the present paper STOC was applied to tsunami calculation to represent the 3D phenomena around the open mouth of tsunami breakwaters. We found net dynamic pressure making up more than half of the net total pressure on the superstructure of submerged breakwater. Thus we can calculate 3D distribution of fluid force on structures considering not only hydrostatic but also dynamic pressures by use of STOC.