A huge box-shaped floating structure is now being proposed as one of the candidates of an offshore airport. According to the plan, the length and the width will be 5000 m and 1000 m respectively while the thickness of the structure is expected to be about 5 m. But, never in history has there existed such a structure, so we have many unknown factors about the behaviors of such structures in waves. Among them, one that is unique to such structures is large elastic deformations that will be induced by the dynamic effects of waves, because the horizontal dimension is extraordinary large compared to its thickness. Experiments on elastic deformations of such structures are quite difficult because, if both the horizontal and vertical dimensions are equally scaled, the resultant model should be as thin as less than 10 mm. Hence, the development of theoretical techniques to be able to predict elastic behaviors without experiments is quite important. For the structures considered in this study, the local displacements due to elastic deformations may be comparable to or even larger than the displacements due to rigid-body motions. Therefore in the predictions of hydrodynamic forces on the structure, the elastic deformations should be taken into account. On the other hand, the elastic deformations are in turn affected by the hydrodynamic forces. Thus the elastic deformations and the hydrodynamic forces interact with each other, which is called a hydroelastic interaction. The main purpose of this paper is to propose a new theoretical technique that can incorporate such hydroelastic interactions in the predictions of dynamic behaviours of a pontoon-type huge floating structure in waves. We compare the numerical results based on the presented theory with experimental data, which are obtained by making use of a flexible model in a water tank and discuss the effectiveness of our theory.