When a plate enters the water, cavity forms around the plate. J.W. Glasheen and T.A. McMahon (Phys. Fluid, 8 (8), 1996; Journal of Experimental Biology, 199, 1996) suggested equations for duration of a cavity around a circular plate and for hydrodynamic force acting on the circular plate. We confirmed the reliability of these equations by two kinds of experiments. Furthermore the analysis by using these equations showed the robot having four legs can support its weight on the water surface when the diameter of the legs and the stroke and frequency of the legs' motion are several times as large as their present values.