Model experiments have been carried out to investigate hydraulic characteristics of dual cylinder caisson on a rubble mound, which is under development as a breakwater in a rough deep sea.
　In the present note, the characteristics of wave reflection, transmission, wave force, and sliding of caisson are described.
　The opening ratios of the permeable outer wall are denoted as εf and εr for the fore and rear sides. Permeable type (εf=0.25, εr=0.10) and wave dissipating type (εf=0.25, εr=0) are tested.
　The reflection coefficient is 0.4～0.6 for both types. And the transmission coefficient is 0.2～0.4 for permeable type, on the other hand, for wave dissipating type, it is similar to the composite breakwater.
　Wave force on each structural element becomes maximum at the different time, and each wave force is smaller than the wave force on ordinary vertical wall.
　In the calculation method, total wave force of dualcylinder caisson is almost same as the wave force of rectangular caisson armored with wave dissipating blocks predicted by Goda's formula (λ=0.8).