A three-dimensional fluid-structure analysis model, which consists of a computational fluid dynamics tool “OpenFOAM” and a structural model “SeanFEM”, was proposed in this study to discuss seawall failure mechanism caused by tsunami. The utility of the model was verified by applying it to the failure phenomenon of seawall in Kojirahama Fishing Port broken by the 2011 Tohoku Earthquake Tsunami. It was found from the numerical result that wave pressure rapidly increases after the tsunami hit the seawall. Moreover, the numerical results showed that the seawall turns over toward land in a similar way to the video images of the seawall failure taken at the port during the tsunami attack. Therefore, the model can be said to have a potential for examining the failure mechanism of coastal structures.