Shaking table tests were conducted to investigate characteristics of hydrodybynamic pressure acting on a double cylindrical caisson. This caisson is composed of a permeable outer cylinder whose upper part has a number of openings and a impermeable inner cylinder.
　The size of caisson model made of acrylic boards was 50cm in heihgt and 50cm in diameter. The opening of outer cylinder were made within the 20cm limit below the top of the model. Opening ratio of 10% and 25% were used. These models were called as a permeable type. The model which has a permeable half-circle in front side of cylinder and a impermeable half-circle in rear side was made and called as a wave dissipating type. In the experiments, hydrodynamic pressures were mea-sured under the conditions of single cylinder type and cylindric wall type.
　In case without opening, the resultant force of hydrodynamic pressure acting on the cylindric wall measured here was almost equal to the value of Westergaard's formula for the wall whose width is the same as the diameter of outer cylinder.
　In case with openings, the resultant force of hydrodynamic pressure on the permeable type model with 25% opeing was about 0.7 times of that on the model without opening. The ratio of resultant force on the wave dissipating type model wioth 25% opening to that on the model without opening was above 0.9.