The authors developed a reduction system for low-frequency ship motions that can effectively reduce low frequency motions of moored ships, and they also proposed the system as one method for countermeasures against long-period waves. The reduction system for low-frequency ship motions can reduce the surge motions of moored ships by preventing resonance with long-period waves, and computer-controlled hybrid mooring winches make it possible to forcibly change the natural period of mooring system consisting of a ship and mooring ropes. In this study, the authorsv erifiede ffectivityo f the reductions ystemf or low-frequencysh ip motionsb y the fundamentahl ydraulic model experiment