In this research, we propose a novel ankle-foot assist device for rehabilitation. This device uses a Stewart platform mechanism to measure and assist the movements of a human ankle joint in six DOF. The Stewart platform mechanism adapts to the displacement of the rotation axis of a human ankle joint during the movement of a human foot. In this study, check the accuracy of motion measurement of this device and the performance of motion control of this assist device by conducting static measurement and motion reproduction experiments. The translation error and The rotation error were less than 0.5 [mm] and 0.6 [deg] respectively. The mean reproducibility of all subjects was 0.98 These experimental results show the validity of our proposed assist device.