The purpose of this study is to develop a simple back assistive device that generates sufficient assistive moment and is not subject to resistant forces while walking. It is widely known that many labor workers suffer from low back pain. To diminish their low back pain, development of a simple and practical back assistive device for use in the workplace is highly recommended. Some simple back assistive devices using elastic elements have been proposed. However, under walking conditions, elastic tension generated by flexion of the hip joint exerts a resistant force against walking. Since the majority of physical workers in industries or other fields have to walk while performing tasks, they require a back assistive device that has sufficient assistive moment in a bending forward posture and does not generate a resistant force while walking. The problem is how to decrease the undesirable resistant force from elastic elements caused by walking, while maintaining a back-supporting property in bending forward tasks. Focusing on the reciprocal motion of legs in walking, we devised a simple mechanism by which elastic tension is diminished under walking conditions. On the other hand, the tension of elastic elements is available in bending forward posture. Making use of the reciprocal motion of legs, two swing arms mounted on the device rotate synchronously with legs. Using this mechanism, elastic tension is almost completely eliminated under walking conditions. The swing arms rotate in the same direction under walking condition, but they cannot rotate in the opposite direction, so that elastic elements are stretched to produce an assistive moment for the back in a bending forward posture. The device proved to be effective in reducing undesirable tension in the lower belt under walking condition. The tension of elastic elements caused by walking decreased to less than one-quarter compared with that of conventional back assistive devices.