We have developed a waist power assist device to support the welfare and nursing operation. In order to realize the human friendly configuration, a soft and light-weight pneumatic rubber artificial muscle is used as an actuator. In this paper, we describe the outline of developed assist device and its fundamental characteristics. Then, the power assist effect is shown through the experiments of lifting the weight and of supporting standing up motion of human. As a result, the developed device can be used as a practical waist power assist device in nursing and various fields.