Provided are an electrolyte-circulating battery in which electrolytes are unlikely to be oxidized and are easily cooled, a heat exchanger in which a corrosive liquid flowing through the inside thereof is unlikely to be oxidized and is easily cooled, and a pipe in which a corrosive liquid flowing through the inside thereof is unlikely to be oxelectrolyte-circulating batteryidized, and which is suitable for cooling the corrosive liquid. The electrolyte-circulating battery includes a battery cell and a circulation passage configured to circulate an electrolyte into the battery cell. The circulation passage includes a complex duct, and the complex duct includes a tubular main body composed of a resin and an oxygen block layer disposed on a periphery of the main body and composed of an organic material that has a lower oxygen transmission rate than the main body.