The South-Pacific region including Indonesia and Papua New Guinea (PNG) has suffered from so many tsunami disasters these days. Tsunami caused at the Sissano area, PNG in 1998 run up to 15m above sea level and killed more than 2000 residents. The north-east region in the Sulawessi island had a tsunami attack with the height of 2～4m and the regional costal houses were heavily damaged by run-up tsunami flow pressure. The fast evacuation from the coast after shaking is the most important regional countermeasure method to tsunami disaster. The regional meeting and lectures on tsunami mechanism have been held in the target areas since the 1998 PNG tsunami event. The elders and children have, however, the difficulty in evacuating from the shore in a few minutes. The warning for early evacuation is not given in case that the tsunami may be generated in the offshore far from shore.
　The alternative countermeasure applying the hard barriers like coastal dikes and detached breakwaters becomes necessary in the coastal region with high tsunami risk. We propose the greenbelt barrier instead of the hard system like breakwaters because they cost high expense. The greenbelt is composed of the tropical trees with enough stability against tsunami pressure and it may be grown up in the residential vegetation of the coastal areas.
　The two dimensional experiment was carried out in a tsunami channel for the evaluation of tsunami reduction effect. The chemical porous media was employed to reproduce the coastal forest in the experimental channel. The experimental results demonstrated that the greenbelt had the tsunami reduction effect similar to that in coastal dikes composed of wave energy dissipating blocks.The drag force coefficient in the greenbelt with porous media was derived from the experimental current and pressure data.
　The numerical simulation in non-linear long wave model with the drag force terms was carried out to study on the greenbelt applicability in a practical topography. The 1998 PNG tsunami was employed for the target. The underwater landslide may be caused in the same time of earthquake shaking. The greenbelt 100m wide was installed along the shore in the Sissano region. The maximum tsunami run-up height on shore became smaller in the case with greenbelt countermeasure than in the case with no protection. The numerical test results demonstrated that the greenbelt with specified tropical trees was applicable as a sustainable tsunami prevention method in the South-Pacific region.