Tsunami is brought about an earthquake in the bottom of the sea. Tsunami's wave height becomes higher in shallower sea. Under that condition, Tsunami is considered as shallow water long period wave. Wave load from Tsunami has much influenced to the motions and mooring tensions of a floating structure in offshore area. For this reason, wave load from Tsunami is essential to planning protection against calamities. Since, an estimation method on Tsunami wave exciting forces in the stage of basic design of floating structure has been presented in this study. Summaries of this study are shown as follows; 1) A concept of solitary wave corresponding to Tsunami has been proposed. Tsunami wave exciting forces can be calculated with lessor labor. 2) The effects of Tsunami wave loads have been made clear comparing two kinds of calculation results. The former is calculation results with Tsunami, the latter is calculation results with wind waves.