This study is an investigation of the mechanism of the washout of bridge girders by tsunamis, performed by conducting 1/10 scale model experiments and numerical calculation for the case of the Numata Overpass. First, the flow experiment conducted. It confirmed that the flow velocity below the bridge girder is higher than the flow velocity above the bridge girder, applying downward force on the bridge girder. Therefore, the result was that the bridge girder was stabilized by increasing resistance to the flow. Next, the solitary wave experiment confirmed that a lift force caused by fluctuation of the water surface washed out the bridge girder. Finally, CADMAS-SURF 3D replicated the experiments numerically and confirmed the validity of the calculations.