A redox flow battery includes: a positive electrolyte storage tank; a negative electrolyte storage tank; a cell stack; a positive electrolyte outward path that sends positive electrolyte to positive electrode chambers in the cell stack; a positive electrolyte return path that sends positive electrolyte to the positive electrolyte storage tank; a negative electrolyte outward path that sends negative electrolyte to negative electrode chambers of the cells; a negative electrolyte return path that sends negative electrolyte to the negative electrolyte storage tank; an entrance open circuit voltage measuring portion that measures an upstream open circuit voltage between the positive electrolyte inside the positive electrolyte outward path and the negative electrolyte inside the negative electrolyte outward path; and an exit open circuit voltage measuring portion that measures a downstream open circuit voltage between the positive electrolyte inside the positive electrolyte return path and the negative electrolyte inside the negative electrolyte return path.