Random wave experiments have been carried out to investigate hydraulic characteristics of a double-cylindrical caisson breakeater, which was selected as a reasonable structure in the rough deep sea. In this note, the calculation method of design wave forces and the results of trial design are presented as well as the description of general characteristics of wave reflection, transmission and wave force.
　The structure tested in the experiments is a caisson of double vertical cylinders fixed on a rectangular base caisson. The upper section of outer cylinder having a number of opening is permeable for one third of the total height and forms a chamber of doughnut shape bounded by the impermeable inner cylinder. The different opening ratio of the permeable outer wall can be selected appropriately for the fore and rear sides.
　The trial design is made for the depth of 50 m. For the design significant wave height of 9 m, the PC- caisson having the outer cylinder of 40 m in the diameter is stable against sliding with safety factor of 1.2, if 35% of total volume excluding the wave chamber is filled by sand.