In the higher mode vibration of giant tankers the transverse section of the ship deforms elastically, and the side shell and the longitudinal bulkhead show the individual deflection, as explained in the first reports). In this report, the hydrodynamic inertia has been investigated theoretically for such a case, and the results shows that the hydrodynamic inertia has three components, i.e., added mass of water for longitudinal bulkhead member, that for side shell member and the hydrodynamic coupling term. Theoretical estimation of these three components nearly coincides with the experiment. The calculation of the natural frequency of the ship using the hydrodynamic inertia derived above agrees with the experiment in a full scale ship with 5% of errors. The calculation of response to a sinusoidal force has been also carried out, the results of which show that, although the calculation agrees with experiment qualitatively, more study is necessary to acquire the precise result.