Recently wearable power assist robots have been developed in order to solve labor shortage and reduce work load. It is desirable that the wearable power assist robots should be lightweight and flexible from the viewpoint of reducing burden on wearers and maintaining safety. Pneumatic actuators are used in various wearable power assist devices because of high power-weight ratio and flexibility. The authors had developed the portable air supply system for retrieving and re-compressing exhausted air using the variable volume tank constructed with elastic material. In addition, the authors had developed the hollow cylindrical-shaped variable volume tank with the reduced internal volume to downsize and improve energy characteristics. The purpose of this study is to construct deformation model to design the hollow cylindrical-shaped variable volume tank with desired characteristics. The effectiveness of the proposed model is verified by comparing the results of the calculated and the measured values. Finally, the proposed model is applied to the multi-layered hollow cylindrical-shaped variable volume tank.