In order to investigate the instability mechanism of caisson-type composite breakwater under tsunami condition, a scale of 1/100 model experiment was performed in laboratory. Loading tests were also carried out to investigate the reduction of bearing capacity under the existence of seepage flow. From the results of laboratory experiment and theoretical analysis, it was confirmed that the bearing capacity of rubble-mound can considerably decrease due to the tsunami-induced seepage flow. It is concluded that the effect of seepage flow in the rubble-mound should be taken into account when making a design of the caisson-type composite breakwater against tsunami.