Few hydrodynamic vibration damper have been developed. As far as the authors know, one is the U-type hydrodynamic vibration damper developed by the NPL of England, and the other is the spring and hydrodynamic damper devised by the authors. The former is the prototype damper for the actual war ship, and the latter is model. Recently Bauer showed that rectangular container filled with two immiscible liquids is very available for reducing vibration. This paper deals with experimental and theoretical investigation into a method to reduce vibration with the sloshing in a tank. The authors carried out exciter test of the off-shore structure model with the rectangular tank filled partially with water, and also calculated damper effect of the sloshing with the theory, in which the sloshing is simplified into mass-spring systems. The authors obtained the conclusion that the sloshing works as dynamic damper for reduction of vibration and the above-mentioned theory, which was developed for estimating dynamic strength of a tank on land, gives coupled natural frequency in good agreement with measured one.