A fluid-storing portion 4 includes an external container (110), an internal container (120), and a coupling material (140) which forms an internal space (130) shielded from the outside between the internal container (120) and the external container (110). When the volume of the internal container (120) is decreased, the internal space (130) is depressurized and receives force in a direction toward the internal space (130) from outside. Consequently, air flows into the internal space (130) from the outside by the action of a runoff prevention mechanism (144 and 147) of the coupling material (140).