PRESSURE FLUCTUATION PREVENTION TANK STRUCTURE, ELECTROLYTE CIRCULATION TYPE SECONDARY BATTERY, AND REDOX FLOW TYPE SECONDARY BATTERY

A simplified, pressure-variation preventing tank structure capable of preventing pressure variations in a gas phase portion resulting from temperature variations, without bringing stored liquid into contact with air. This pressure-variation preventing structure includes a breather bag (3) arranged in a gas phase portion (2) of a tank (1) and inflating/deflating in communication with outside air, and a manhole (21) to which the breather bag is attached to suspend in a gas phase portion, including a communication hole (22) for the breather bag to communicate with outside air. The breather bag (3) has air-blocking, acid-resistant and expandable characteristics.