Floating breakwaters have been adopted in the comparatively deep sea area with a mild wave climate, such as inland seas. In that area, aquacultural facilities, such as fish ponds, oyster farming facilities and so on, are usually installed. The development of highly dissipative floating breakwaters has been devoted in that sea area for reducing reflected waves from newly constructed breakwaters and sea walls, which are usually adopted for protecting the newly reclaimed land from wave actions. In this study, a new type of floating breakwater intending to reduce both the reflected and transmitted waves was proposed. There are two major dissipation mechanisms presumed in this model. One is the wave slamming loss of the horizontally installed circular cylinders near the water surface in the breakwater. The other is the enhancement of vortex flows around the lower end of side curtain walls arranged with some spacing. Here, the piston mode wave resonance between the two side curtain walls is used for the enhancement. By the use of various sections of the breakwater, especially changing side curtain wall configurations, performance of each breakwater model has been extensively examined.