As one of the several characteristics of hemiplegic patients after stroke, the patients conduct sit-to-stand and stand-to-sit (STS) motion using their unaffected leg. The purpose of this study is to develop a STS motion support system for helping the patients decrease the difference of the usage ratio between the affected side and the unaffected side, and increase the usage of the affected leg. The developed system supports the STS motions by moving the seat linearly. The system also provides the user visual motor information for each leg by measuring the floor reaction force (FRF) during STS motions. With the motion support and the visual feedback, the user can lean how to increase the usage ratio of the affected leg and equalize the usage ratio of the unaffected leg with the affected leg. The STS experiments were carried out in two hemiplegic patients and a quadriplegic patient to confirm the effectiveness of the system. All the participants increased the FRF of the affected leg and decreased that of the unaffected leg by using the system. Even if the system wasn't used, two participants decreased the difference of the usage ratio between the both sides and increase the FRF of the affected leg. We confirmed not only the immediate effect but also the longitudinal effect of the system. These results suggest that the developed system is effective in helping hemiplegic patients to improve STS motions.