A new method to analyze the response of a thin elastic plate of large horizontal size floating in waves is presented. Huge horizontal size and small thickness of it are typical of the recent design of floating airport. A benefit of this new method is that we need not the modal analysis of the body deflection, and the solution of a hydrodynamic boundary value problem and the solution for the body motion including the elastic deflection are simultaneously obtained. Some results of numerical implementation of the method are presented.