The invention provides an operating method of a redox flow battery capable of grasping a charging state of the battery more reliably to stabilize an output capacity of the battery. The method is for operating the redox flow battery comprising a cell stack 1 comprising a plurality of cells. A selected cell(s) in the cell stack 1, to and from which positive electrode electrolyte and negative electrode electrolyte are supplied and discharged and which is/are not normally connected to a DC/AC converter 225, is/are in the form of an auxiliary cell 2 used for measuring a charging rate of the electrolyte. Also, a stop of charge of a main cell 3 and a stop of discharge of the main cell 3 are controlled with reference to a circuit voltage obtained from the auxiliary cell 2. Since the auxiliary cell 2 is integrally incorporated in the cell stack 1, the charging state of the battery can be grasped reliably without stopping the charge/discharge operation of the main cell 3. Also, since the stop of charge of the main cell 3 and the stop of discharge of the same are controlled with reference to the measured circuit voltage, the output capacity can be stabilized.