Wearable robots actuated by air pressure require small air pressure generation system. Miniaturization of air pressure generation system using a pump is limited by size of the pump when the system requires compressing or exhausting air quickly. If the system requires controlling the air pressure quickly but not frequently, it is possible to reduce the weight of the system by controlling air pressure using a tank that is pressurized or compressed by a small pump. However the capacity of the tank must be enough large for the control object especially in case of vacuuming. This paper shows that the system can be miniaturized by pressurizing or decompressing an object in stages using two or more tanks. The theoretical analysis of the relation between the number of tanks and the generative air pressure shows that the number of tanks is optimized to the size and weight of the system. The time to generate an air pressure in stages is estimated by the number of tanks. These analyses were confirmed by experiments using vacuuming systems with different number of tanks. This paper also shows that reuse of the air pressure in tanks is performed effectively in this method. The relation between the number of tanks and the effectiveness of the reuse of the air pressure is described.