Scouring due to tsunami causes a decrease in the bearing capacity of structures, resulting in serious damage. The present study attempted to explicate the scouring mechanism focusing on the tractive force and the pore water pressure in the ground. In the case of scouring due to a open-channel flow, the hydraulic gradient in the vertically upward direction was generated along with generation of the excess pore water pressure in the saturated ground and, consequently, scouring was facilitated. Scouring was also expanded when the pore air was blown out of the unsaturated ground. In the case of scouring due to overflow, the excess pore water pressure was generated in the ground due to a collision of water drops against the ground. Consequently, the ground like liquefaction phenomenon occurred and, scouring was facilitated in the ground.