The accelerograms recorded on the large composite type breakwater which is located at a mouth of Ofunato Bay in the water depth of -38m were analyzed. And the validity of coupled hydrodynamic response calculation method were discussed by means of the accelerograms. Comparing the accelerograms of the caisson's top with those of the base rock, the top of the caisson shows clear responses. The response ratio of the caisson's top to the base rock decreased with the increase of the maximum acceleration of the base rock. According to the transfer function between the caisson's top and the base rock, the frequanecy of the first order which showed the peak of the transfer function decreased with the increase of the base rock acceleration. This seismic behavior was due to the nonlinear characteristics of the mound's material. The comparison of the coupled hydrodynamic response using the equivalent linear method with the observed acceleations showed a relatively good agreement. And the estimated strain-dependent characteristics of the mound's material was almost equal to the cyclic triaxial test results of the gravel.