A movable tsunami buffer dam has a unit configured such that a plurality of separate units, each of which having a shape in which a frame made of a light material is sandwiched by a pair of plates made of a light material, is stacked with said plates disposed in a pile; and a locking member for locking said unit to a ground surface such that said unit can rise up from said ground surface and collapse onto said ground surface. The separate units has a structure in which water from a tsunami can advance into a space formed between said plates by said frame.A required thickness is ensured due to said unit being configured such that said separate units are stacked, and the manufacturing cost is reduced to a greater extent than in the case of a dam configured from a single separate unit of said required thickness, because the big size lumber for obtaining the units is very expensive. The unit is installed in a state of being collapsed on said ground surface at normal times, and when a tsunami arrives, said unit rising up due to the force of the tsunami and the buoyancy of the seawater, resisting the passage of the tsunami and reducing the power of the tsunami.