This report describes the results of the large scale model tests on the buoyancy-driven vertical piling breakwater by using the buoyancy-driven piles. This breakwater consists of rows of steel pip. piles and each pile consists of upper and lower steel pipes. The overall performance of this breakwater has been verified, but the relation between stress and wave power is not cleared at the part of lap joint between upper pipes and lower pipes, at which large stress may be occurred. So, the large scale model tests by using the buoyancy-driven piles were conducted. Main results of this paper are that movement of the upper pile under waves is confirmed and the relationship between the strain at the lap part and the wave power is clarified qualitatively.