Rivermouth blockade has been occurred for a long period at Monobe River of Shikoku-island in Japan. The rivermouth blockade causes serious environmental problems for river control and ecosystem such as spawn of sweetfish. In this study, amount of sediment discharge from upstream is analyzed by surveying the morphology data of Monobe River for 32 years (1973-2005) and by means of aerial photographs to find the causes of rivermouth blockade. In addition, numerical simulation by 3D k-ε turbulence model and sediment discharge formulas by Van Rijn and Sunamura are applied for simulating the bed variation of sand bar near rivermouth considering with the flood/normal discharges, flow depth, wave direction, wave height and period of ocean wave. The phenomena of formation and destruction of the sand bar are reasonably simulated for conditions of winter and summer seasons.