The vibration of a ship's bottom may be considered to be a combination of two factors; the one, which is fundamental, derived from vibration of a rigid body and the other originating from the flexuous deformation of the bottom. An acrylite plate framed by a wooden block was vibrated in the water, and the reactive force was electrically measured by a load cell. The added mass caused by the vibration of the rigid plate is invariable through the given frequency range in the experiment. In the vibration of the flexuous bottom, the inertia force of a fluid is mechanically similar to the reaction which a mass-spring system gives to the base. A theoretical analysis was attempted on this simplified model, and it is shown that the reaction and the added mass caused by the vibration of the partial flexure in the water can be calculated.