The Indian Ocean Tsunami in 2004 induced gigantic tsunami hazards in the coastal areas. Meanwhile the effects of coastal forest (greenbelt) to mitigate the loss of human being and coastal villages were broadcasted and the tsunami prevention employing the greenbelt was paid attention especially in the South Asian countries. A few experimental results in two-dimensional channels demonstrated the fundamental effect of greenbelt against tsunami flows. The practical technique to determine the appropriate arrangement of greenbelt, however, has not been discussed. In the paper the variation of tsunami force due to the greenbelt with uniform and leap-frog arrangement is numerically calculated and the tsunami flow velocity in a gap between the neighboring greenbelts is evaluated. Finally the effect of greenbelt in a model coast approximating the prototype is estimated.