PURPOSE: To provide a hollow wave-breaking block which is excellent in the wave-breaking effect and which has an effective attenuation function of undulation energy and a wave-breaking structure by use of the blocks.

CONSTITUTION: Hollow wave-breaking blocks 10 in which the side faces are fully opened and a plurality of openings are provided at the upper face so as to range 5-25%, preferably 10-20%, in the opening ratio, are laid on a foundation mound 12 prepared on the bottom of the sea. The stream due to reaction is moved up and down through the opening of the upper face to attenuate the vertical undulation energy. And further, the horizontal stream of the seawater can pass through freely and the increase of the average water level is prevented.