We developed a new system to assist the elderly with walking. Previous walking assist systems directly control the hips or legs. The proposed system provides walking assistance using passive dynamic walking by controlling trunk rotation. We verify the relation between trunk rotation and walking by fixing trunk rotation. Gait analysis was performed using a motion-capture system and electromyography. We propose an assistive device for a new form of walking assistance. We also developed a prototype of the assistive device and verify its effectiveness.