In this study, we developed a gripper capable of gripping a soft object with low pressure. At disaster sites, it is necessary for robots to grip various kinds of objects. The universal gripper (UG) is an example of a gripping tool, being a soft gripper that exploits the jamming effect of grain inside an elastomer bag. However, the UG cannot fit to objects precisely, because it is filled with coffee granules. Consequently, strong pressure must be used to fit the gripper onto an object. As one approach to overcoming this problem, a method to change the grain quantity is required. Therefore, we have devised a variable-inner-volume mechanism using an air cylinder. The grain quantity changes relative to changes in the inner volume, allowing the variable-inner-volume gripper to grip various objects. Herein, we also measured the pressure necessary for object gripping.