In this paper, we control posture of end plate of motion reduction device with under-actuated parallel-link mechanism. This device forms 3-DOF planar parallel link and six-joint link mechanism equips only two linear actuators. Using of under-actuated parallel-link mechanism reduced number of actuators compared to general parallel-link mechanism. We propose whole system including ship and land with motion reduction device to improve stability under gravity and facilitate control. Whole system is possible to reduce sway of 3-DOF of the ship by two linear actuators. The effectiveness of motion reduction device is shown by simulation results and experimental results in small scale experimental equipment.