Rubble mound breakwater is a coastal structure that defends the coastal area by dissipating wave energy when an incident waves passes in the structures. Several studies have been made on reflection and transmission coefficient, there is little agreement on the energy dissipation in rubble mound breakwater. The purpose of this study is to examine the energy dissipation in rubble mound breakwater, the authors carried out the experiment of three different scales. The main conclusion is shown below.
1) Energy dissipation has a liner relationship with the wave steepness of incident waves, and the results of the irregular wave experiment can be evaluated by regular wave experimental results.
2) The transmission coefficient can be evaluated by the empirical formula by Kondo and Takeda (1983).
3) A clear influence did not appear for the energy loss, reflection and transmission coefficient though the experiments on three different scales were compared in this research.
　Moreover, the following conclusions were obtained by applying the three dimensional numerical wave tank.
　The drag force coefficient is greatly influenced the profile of water surface behind the rubble mound. In this study, when the value in which the recommended value is doubled is used, the experimental data can be reproduced well.