Calm sea area is necessary for nursery, marine leisure, renewal of harbour, and so on. For those purposes, floating breakwaters have been taken interest from the view of cost, preservation of water, etc. Especially, reasonable-sized ones suited to waves of long period are desired. In this study, the floating breakwaters utilizing inside water are treated. A proper design of the mode of water behavior is expected effective to reduce the transmitted waves. It is because the radiation waves from body are interactive with relative motion of the body and inside water. At first, a theoretical method is developed to evaluate the breakwater performance. Its validity is confirmed by comparison with experimental results. Secondly, in order to extend the efficient range of wave period, a novel idea of the mechanism for additional mode of water behavior is introduced into the existing concept of floating breakwater. That is, inside winding flow mode with controlled long natural period affects the sway-roll motion in the vicinity of the matched wave period. Finally, a proto-type is designed by the use of the theoretical method, and good performance of the newly developed floating breakwater is confirmed for longer wave period.