The field survey and the literature study on the control forest conditions are carried out to evaluate the effect of actual coastal forest and to find the condition of coastal forest. The relation between the forest density and the diameter of trunks is obtained through the measurement of control forests and some previous researches, which can be useful information for the evaluation of effects. The numerical simulation including the model of control forest is performed for evaluating the quantitative effect for tsunami reduction and damage. The table to estimate the reduction effect in terms of wave height, velocity and hydraulic pressure is obtained and can be compared the Shuto's tsunami damage criteria. Finally, the literature study was done to recognize the mitigation effect of tsunami disaster by the coastal forest in the past. Four main functions to reduce tsunami disaster are found; 1) to stop drifts or ships carried by a tsunami, 2) to reduce tsunami energy, 3) to form sand dune protecting tsunamis as well as high waves, and 4) to catch people carried back by a tsunami to the sea. However, it should be reminded that the coastal forests would be destroyed by a huge tsunami and would be floating material to cause secondary damage.