Walking assistive device is gaining attention, because of aging society these days. To assist their rehabilitation and realize their independent walk, the field of wearable device, such as exoskeleton is being researched. This paper mainly focuses on the device for patients with mild hemiplegia, caused by stroke. Many walking assistive devices were researched so far. However, these devices restrict patients' natural motion due to the structure of the device. Moreover, conventional control system has not satisfied the balance between assistance and patients' will. Therefore, the mechanism of new assistive device which reduce the restriction of the knee joint is proposed. This exoskeleton lifts up their lower limb by one link and impedance control considering the walking characteristics of the patient are also proposed. The controller focuses on the walking phase, floor reaction force and reaction force of each joint. After discussing walking characteristics of healthy person and patient with hemiplegia, proposed control is introduced. Effectiveness of proposed exoskeleton is confirmed by simulation.