Dealing with the strength of ships in bad weather, we must know the external forces acting to ships. Especially the problem of pounding is very important. As the premise of the problem, we had carried out the following experiment. Fine test pieces made of hard alminium alloy 56 S (2mm thick, 11.3-22. 6cm diameter) were strengthened their rigidity with wood plates (about 2cm thick). They were fixed on the falling part of the testing machine (newly designed), and were fallen upon the water surface from 2.5-20cm height. We had measured the impulsive force using piezo-electromagnetic-oscillograph. Consequently we had the empirical formula based on the following assumption : The maximum impulsive force of falling flat plate consists of two parts, one which is proportional to√ H has the elastic character, and the other which is proportional to H had the hydrodynamical character. That is P _max=α√2 kgMm / M + m √ H +βρ gFH where P _max : The maximum impulsive force α : Empirical constant 0.7 (non-dimension) k : Spring constant of falling body m : Virtual mass of fluid M : Mass of falling body g : Acceleration due to gravity H : Falling height β : Empirical constant 18 (non-dimension) ρ : Density of fluid F : Area of flat bottom