Many kinds of power assist device have been developed, and are driven with various actuators such as an electric motor, a hydraulic cylinder and so on. By using the exoskeleton, a high generated torque actuator can be introduced. An assist performance of these devices becomes high. On the other hand, a realization of exoskeleton which has the same D.O.F of a human is not easy from considerations about a size and strength of device. In this study, the power assist wear for lower limb is developed. The developed wear is like trousers. A human can be assisted by just wearing trousers on which the pneumatic soft actuators are put. The pneumatic soft actuator has a high power weight ratio, and has a light weight. These features contribute to realize the simple structure which is like clothes. In this paper, the structure of power assist wear is discussed, and the application of power assist wear to assist going up and down stairs are described.