We have developed the estimation method of the first-order hydroelastic responses of a very large floating structure. The deformation of the structure is small in the frequency ranges of the wind wave, because the wavelengths are very short comparing with the length of the structure. However, the lengths of the second-order long period waves that are the difference-frequency component of the second-order wave are very long. Generally, effects of that wave are very important for the design of the mooring system. But, the hydroelastic responses may be occurred by the second-order long period vertical wave loads. In this paper, we examined the behavior of the hydroelastic responses of the very large floating structure due to the second-order wave forces. Then, the analysis method of the second-order wave loads on elastic modes is developed. The method is based on the pressure distribution method.