A Slit Caisson with Wave Dissipating Blocks is developed for a tsunami prevention breakwater. The caisson has many wave dissipating concrete blocks in a wave chamber that is composed of rectangular columns and a verical wall .Therefore it is necessary to estimate three dimensional distribution of the hydrodynamic pressure and behavior of the wave dissipating blocks for discussion of dynamic stability of the breakwater. In this study, a series of model tests have been conducted to examine characteristics of hydrodynamic pressure and dynamic behavior of wave dissipating blocks during vibration. A three dimensional FEM program (BEAD3D) to analyze water-structure interaction during earthquake was developed and the validity of the calculation method was discussed comparing the test results.
　The following conclusions were obtained:
(1) The hydrodynamic pressure calculated by the BEAD3D was in good agreement with the measured value in model tests.
(2) A design procedure using Westergaard's formula and Zanger's formula was presented for calcualation of the resultant hydrodynamic pressure to act on the caisson.
(3) A difference in vibration between the wave dissipating concrete blocks in the wave chamber and the caisson was not observed in tests. The wave dissipating concrete blocks vibrated together with the caisson.