A redox flow battery (α) comprising: a battery unit which includes a positive electrode, a negative electrode, and a membrane; a positive electrode electrolyte tank which stores a positive electrode electrolyte to be supplied to the battery unit; a negative electrode electrolyte tank which stores a negative electrode electrolyte to be supplied to the battery unit; and a pressure adjustment mechanism which is attached to at least one of the positive electrode electrolyte tank and the negative electrode electrolyte tank and adjusts the pressure of a gas phase inside the electrolyte tank, wherein the pressure adjustment mechanism is provided with a water sealed valve which includes a storage container that stores a pressure adjusting liquid, a first exhaust tube that extends from the gas phase inside the electrolyte tank, extends through a gas phase inside the storage container, and is open to a liquid phase inside the storage container, and a second exhaust tube whose one end is open to the gas phase inside the storage container and whose other end is open to the atmosphere; and the pressure adjusting liquid is a non-volatile liquid at normal temperature and normal pressure and is selected from silicone oil and liquid paraffin, wherein the specific gravity of the pressure adjusting liquid is lower than the specific gravity of water; and the water sealed valve further includes an overflow tube that is open on the side or bottom of the storage container and discharges to the outside dew condensation water accumulated on the lower side of the pressure adjusting liquid inside the storage container. a battery unit which includes a positive electrode, a negative electrode, and a membrane; a positive electrode electrolyte tank which stores a positive electrode electrolyte to be supplied to the battery unit; a negative electrode electrolyte tank which stores a negative electrode electrolyte to be supplied to the battery unit; and a pressure adjustment mechanism which is attached to at least one of the positive electrode electrolyte tank and the negative electrode electrolyte tank and adjusts the pressure of a gas phase inside the electrolyte tank, wherein the pressure adjustment mechanism is provided with a water sealed valve which includes a storage container that stores a pressure adjusting liquid, a first exhaust tube that extends from the gas phase inside the electrolyte tank, extends through a gas phase inside the storage container, and is open to a liquid phase inside the storage container, and a second exhaust tube whose one end is open to the gas phase inside the storage container and whose other end is open to the atmosphere; and the pressure adjusting liquid is a non-volatile liquid at normal temperature and normal pressure and is selected from silicone oil and liquid paraffin, wherein the specific gravity of the pressure adjusting liquid is lower than the specific gravity of water; and the water sealed valve further includes an overflow tube that is open on the side or bottom of the storage container and discharges to the outside dew condensation water accumulated on the lower side of the pressure adjusting liquid inside the storage container.