The deepest breakwater in the world is under construction in Kamaishi Bay in order to protect the port area from tsunamis as well as storm waves. The deepest depth is 63m and a submerged dike with the crest depth of 19m is planned to be built at the central opening section of 300m wide to close the bay mouth as much as possible. Because of this narrowness, strong current of 8m/s is expected at the occasion of design tsunami attack. The submerged dike consisting of a rubble mound foundation and a crown caisson must be stable against the strong current.
　In the present paper, the result of experiments which has been carried out to give the design information is described. The model scale is 1/50 and following subjects are investigated:
a. flow pattern around the opening section due to steady flow,
b. stability of armour units for the submerged rubble mound foundation against periodic flow,
c. hydrodynamic forces and stability of the submerged crown caisson.
　According to the test results, the damage pattern of the submerged rubble mound foundation and the relation between damage ratio and armour weight are disclosed. As for the stability of crown submerged caisson, pressure coefficients are proposed in order to calculate design hydrodynamic forces.