To design a very large floating structure, such as a floating airport, we have to estimate the hydroelastic response on any point of the very large floating structure exactly. We developed the estimating method of the hydroelastic response of a very large pontoon type floating structure. In this method, to obtain the hydrodynamic forces we used the pressure distribution method with Shallow draft assumption, considering the elastic motion of a floating body. The elastic response of very large floating structure was estimated by using the modal analysis of 2-D beam model. On the other hand, to observe the elastic response of a large pontoon type structure we carried out experiments in head sea and head+beam sea condition. The experimental model was a pontoon type large elastic floating body. This model consists of many box type rigid floats that were connected by longitudinal and lateral elastic beams. To measure vertical motions of multiple points of the elastic model we applied the optical and image processing system. We could measure the elastic motions and deformations of the model in 2-directional waves. To validate the corresponding of numerical estimating method we compare the numerical results with experimental results. From these results, we found good agreement and confirmed the accuracy of this numerical estimating method of the hydroelastic of a very large pontoon type floating structure. Furthermore, we discussed the hydroelastic responses and bending moments in shorter wave length with changing stiffness of the floating structure.