The lateral vibration of deep girder in water has been treated in this report since the fatigue due to vibration is supposed to be one of the principal causes of the fractures in the local structures in the cargo tanks. The inertia of surrounding water is calculated by two-dimensional FEM using distributed sources and sinks, and taking account of three-dimensional correction. The effect of both the structure and the hydrodynamic inertia on the lateral vibration is examined theoretically where the latter is expressed in term of kinetic energy for the linear combination of the elastic modes. The effect of the face plate upon the pressure distribution and upon the frequency is shown to be large in water. The model experiments have been carried out and the results fairly agreed with that of calculation. The calculated natural frequency agrees with that obtained on board a large tanker with about 10% of errors and it is shown that the present calculation method is applicable to the actual structures and that the frequencies of this type of structures immersed in water may be lowered enough to cause the resonance with the blade frequency component of the exciting forces.