In Japan, detached breakwaters made with wave dissipating concrete blocks such as Tetrapods have been widely applied, but the effectiveness of such kinds of detached breakwaters on tsunami disaster prevention and stability of the structure have not been discussed in detail. Only few studies on wave run-up reduction were done after Nihonkai Chubu Earthquake Tsunami in 1983. In order to mitigate tsunami disasters in sea coast areas, the effectiveness of detached breakwaters and the stability of wave-dissipating concrete blocks should be properly evaluated. Authors has just studied on the effectiveness of detached breakwater on wave run-up reduction and wave pressure reduction on the seawall taking actually possible damage to detached breakwaters into account (Hanzawa et al., 2011). In this particular study, the hydraulic model tests have been systematically and carefully carried out to discuss the stability of wave-dissipating blocks of detached breakwater against solitary tsunami waves. Tetrapods of rear side of the breakwater tends to be damaged like land slide. The damage by tsunami is bigger than that by ordinary wind waves. A countermeasure for improving the stability is also proposed based on the damage process of detached breakwater.