It has been known that the natural frequency of the circuler cylinder, in which filled with water in the fundamental mode is about one half of the frequency of the empty cylinder. But on the slightly elliptical cylinder the fundamental natural frequency is not only so decreased but in extrem case upper than the frequency of empty cylinder. It is supposed that in such a case the water acts not only as the attached mass but as elastical component, that is to say, in the vibration of the elliptical cylinder the elastical vibration energy increases by the tensile stress caused by uncompressibility of the water in the cylinder. In this study, using the model cylinder the vibration and the pressure of the cylinder surface is measured at the resonance condition in the fundamental mode. The experimental result shows that the natural frequency increases with the elliptical eccentricity and the pressure measured at succeeding untinode position are inphase
