The present paper points out on the basis of the experimental results for the typical example of slided caisson that careful at-tention should be paid to the possibility of the generation of powerful impact breaking wave forces in the design of composite breakwater. The generation conditions are investigated by the two-d imensional experiments and the three-dimensional experiments. It is concluded that the impact breaking wave forces are apt to be produced on the caisson, where the mound is relatively high and has an appropriately broad crest width. But, the impact breaking wave forces are likely to occur only in the case when the dierection of incident waves is near to the perpendicular to the breakwater.
　If the generation of impact breaking wave forces are unavoidable owing to the breakwater configuration, provision of an appropriate wave dissipating work in front of the caisson is recommended to alleviate the wave forces. From this point of view, discussion is given on the method of calculation of design wave forces on the caisson covered with wave dissipating concrete blocks and the wave dissipating caisson with a permeable front wall.