PURPOSE: To improve the wave breaking effect to waves of wavelengths ranging wider than usual by hanging weights from a control tank having a breaking water action to add a gravity pendulum vibration system.

CONSTITUTION: A short wave is broken by the lateral movement of water in a tank 7 in the control tank 7 for a floating break water main body 1. Subsequently, a long wave is broken by the vertical movement of water in the tank 7. The weights 3 and the lengths of chains 4 are suitable set according to the relation of gravity pendulumns each of which comprises a weight 3 hung from the tank 7 through a universal hinge and a chain 4 to the vertical and lateral movement period of water in the tank 7 to set the period of a break water 1 including gravity. Thus, the breaking water effect can be remarkably improved extending over a wide range of wavelength.