Floating structures are generally evaluated assumed to be rigid. A slender structure such as a floating bridge, however, has elasticity that must be accounted for. We developed analysis method that takes structural elasticity into account. The structure, formulated by finite element method (FEM), and fluid, formulated by boundary element method (BEM), interact. An elastic model test in water tank showed that analysis evaluated the influence of structural elasticity properly, confirming the validity of elastic response analysis. A numerical example using this method emphasizes the need to consider the influence of structural elasticity on the dynamic response of the floating bridge to waves.