PURPOSE: To permit tidal waves to be dissipated by a method in which the top portion of waves coming from the outside of harbor is confined and the sea water confined in the bottom portion is dropped by using a breakwater in which a bottomless box partitioned inside is fixed on the average sea level.

CONSTITUTION: The lower end of a bottomless box 2 having vertical and horizontal partition walls 1 inside is fixed to supports 3, e.g., column, etc., erected on the seabed by positioning it on nearly average water surface level. The wall 1 on the inside of harbor of the box 2 has a horizontal plate 4 extending deep into water and horizontally extending toward the inside of harbor, and the top plate 6 of the box 2 has a check valve 7 through which air in each chamber 5 in the box 2 is allowed to escape. When waves come from the outside of harbor, the top portion of waves is confined in the chambers 5 while removing air in the chambers 5, and when the bottom portion of waves comes-in, air enters the chambers 5 from below and the sea water being confined is dropped. Waves can thus be dissipated.