Recently declining birthrate and aging population have been causing serious labor shortage. In order to reduce the burden on workers and improve productivity, devices such as wearable power assist robots have been developed. Pneumatic actuators are used in various wearable power assist devices because of their high power‐to‐weight ratio and flexibility [1]. To drive the actuators, air supply systems are required. The air supply systems are composed of air compressors, tanks, valves and various sensors in most cases. For usability and maneuverability in practical use, the air supply systems are expected to be portable as well as the wearable power assist devices themselves. Okui et al. proposed a portable power source using chemical reaction of sodium bicarbonate and citric acid [2]. Kitagawa et al. suggested an air source by using dry ice [3]. The authors have developed a portable air supply system for retrieving and re‐compressing exhausted air using a variable volume tank [4]. Conventional air supply systems have employed constant volume tanks. If a small constant volume tank is used in order to downsize the system, inner pressure of the tank drops significantly when compressed air is supplied to the actuator. In contrast, the variable volume tank expands when compressed air flows in, and by storing part of the energy of compressed air as elastic energy, it decreases the amount of pressure change. This effect is called pressure relaxation characteristics. However, physical properties of elastomer affect the characteristics of the variable volume tank. Therefore, to increase relaxationpressure, the tank became thick and heavy. Although a double layer variable volume tank [5] was developed to increase storable pressure without increase in thickness and weight, it became large. In this study, a hollow cylindrical‐shaped variable volume tank is developed as a solution to this issue. This paper describes the effect of decreasing pressure drop based on the pneumatic energy characteristics when compressed air is stored in the developed hollow cylindrical‐shaped tank. We also explain the feasibility of achieving the desired energy characteristic at high pressure by multilayering the tanks to increase outer pressure.