This paper reports on the investigation of coupling effect between beam vibrations and oval vibrations on a cylindrical water storage tank, which are excited by large amplitude sinusoidal waves. Some large amplitude sweep tests with the different magnitudes of input acceleration have conducted for a test tank. Their transfer functions showed that the predominant frequency of resonance curve had shifted to lower frequency region and the magnification factor had gotten smaller as the magnitude of input acceleration increased more. These results of vibration tests have proved that the coupling effect between beam vibrations and oval vibrations reduced the flexural rigidity of the beam vibration of a tank.