Features of numerical errors of Tsunami simulation using unstructured triangular meshes with finite volume methods investigated quantitatively. A series of test simulations of the Tsunami wave propagating in thedee(are) p area shows that the numerical diffusion errors strongly depend on the spatial size of mesh, kind of the adopted numerical scheme and the travel distance to be solved. Furthermore, introduction of an Adaptive Mesh Refinement (AMR) technique to the triangular unstructured mesh system is investigated. By conducting numerical simulation of the Tsunami propagation for 2011 Tohoku Earthquake, it is found that the AMR technique can suppress the numerical errors and reduce the required computational loads.