We have carried out three-dimensional numerical simulations of a tsunami passing through a structure to investigate the characteristics of the hydrodynamic force induced by a tsunami. In the simulation, an idealized tsunami was used, which has a sharp head and a constant flow velocity and a constant flow depth in the upstream region. The results show the water level on the front face of the structure can be estimated by using the specific energy of the upstream region of the structure. In addition, the hydrodynamic force acting on the front face of the structure is approximated as the hydrostatic force with the water level of the front face of the structure. Thus, this algorithm enables us to estimate the hydrodynamic force acting on the front face of the structure by using the specific energy predicted by tsunami inundation simulations.