In coastal construction, we have developed a wave absorbing breakwater using buttress in order to secure a desired work space with reduced wave height and secure a stage of heavy machinery etc. to be used for the work space on the sea. It is an earth retaining structure that buried a buttress which is an L-Type wall structure in the rubble mound breakwater, and stabilized against sliding and overturning by the weight and frictional resistance of the rubble stone. By doing like this, the side of the rubble mound breakwater rides vertically, and the stage of heavy machinery can be constructed close to the work space. In order to contribute to the design of this wave absorbing breakwater using buttress, hydraulic model experiment and numerical analysis were carried out to confirm basic hydraulic performance. First, we conducted a hydraulic model experiment assuming a general steep coast. In order to confirm the effect of the wave-dissipating blocks, experiment was conducted even in the state where the wave-dissipating block model was not installed. The sampling frequency was 20 Hz for the water surface elevation and 200 Hz for the wave pressure to the buttress model. Furthermore, we measured the overtopping quantity to the hinterland of the buttress model, and recorded the shape of the breakwater body by image analysis. Secondly, we conducted a numerical analysis to reproduce experimental results using CADMAS-SURF/3D. As a result, it became clear that the engaging effect between the bottom slab beam and the rubble stone worked, the uplift pressure is very small, and the horizontal wave force reduction coefficient in Goda’s formula can be applied at the maximum simultaneous wave pressure action. And, we clarified the estimation method of the wave force and the wave overtopping rate, and the deformation characteristics of the rubble mound breakwater.