A redox flow battery system comprising: a battery cell; an electrolyte tank; circulation piping through which an electrolyte is supplied from the electrolyte tank to the battery cell by circulation; and a pump that causes the electrolyte to circulate through the circulation piping, wherein the redox flow battery system further includes a pump controller that is configured to control a flow rate of the pump, a terminal voltage measurement unit that is configured to measure a terminal voltage of the battery cell, and a measurement unit that is configured to measure at least two parameters selected from among an inlet-side state of charge of the electrolyte supplied to the battery cell, an outlet-side state of charge of the electrolyte drained from the battery cell, and a charge/discharge current input to/output from the battery cell, wherein the pump controller includes a pump flow-rate computation unit that is configured to calculate a charge/discharge efficiency of the battery cell from the at least two parameters measured by the measurement unit and, based on the charge/discharge efficiency, to determine the flow rate of the pump so that the electrolyte drained from the battery cell is neither overcharged nor overdischarged, the charge/discharge efficiency of the battery cell representing an amount of change in a state of charge of the electrolyte occurring while the electrolyte passes through the battery cell, a pump flow-rate instruction unit that is configured to set in the pump the flow rate determined by the pump flow-rate computation unit, and a terminal voltage determination unit that is configured to determine whether or not the terminal voltage of the battery cell reaches an upper limit or a lower limit of a specified voltage range, wherein the pump controller is configured such that when the terminal voltage reaches the upper or lower limit of the specified voltage range, the pump flow-rate computation unit determines that the flow rate of the pump is increased by a specified amount, and when the terminal voltage does not reach the upper or lower limit of the specified voltage range, the pump flow-rate computation unit calculates the charge/discharge efficiency of the battery cell from the at least two parameters measured by the measurement unit and, based on the charge/discharge efficiency, determines the flow rate of the pump so that the electrolyte drained from the battery cell is neither overcharged nor overdischarged. a battery cell; an electrolyte tank; circulation piping through which an electrolyte is supplied from the electrolyte tank to the battery cell by circulation; and a pump that causes the electrolyte to circulate through the circulation piping, wherein the redox flow battery system further includes a pump controller that is configured to control a flow rate of the pump, a terminal voltage measurement unit that is configured to measure a terminal voltage of the battery cell, and a measurement unit that is configured to measure at least two parameters selected from among an inlet-side state of charge of the electrolyte supplied to the battery cell, an outlet-side state of charge of the electrolyte drained from the battery cell, and a charge/discharge current input to/output from the battery cell, wherein the pump controller includes a pump flow-rate computation unit that is configured to calculate a charge/discharge efficiency of the battery cell from the at least two parameters measured by the measurement unit and, based on the charge/discharge efficiency, to determine the flow rate of the pump so that the electrolyte drained from the battery cell is neither overcharged nor overdischarged, the charge/discharge efficiency of the battery cell representing an amount of change in a state of charge of the electrolyte occurring while the electrolyte passes through the battery cell, a pump flow-rate instruction unit that is configured to set in the pump the flow rate determined by the pump flow-rate computation unit, and a pump flow-rate computation unit that is configured to calculate a charge/discharge efficiency of the battery cell from the at least two parameters measured by the measurement unit and, based on the charge/discharge efficiency, to determine the flow rate of the pump so that the electrolyte drained from the battery cell is neither overcharged nor overdischarged, the charge/discharge efficiency of the battery cell representing an amount of change in a state of charge of the electrolyte occurring while the electrolyte passes through the battery cell, a pump flow-rate instruction unit that is configured to set in the pump the flow rate determined by the pump flow-rate computation unit, and a terminal voltage determination unit that is configured to determine whether or not the terminal voltage of the battery cell reaches an upper limit or a lower limit of a specified voltage range, wherein the pump controller is configured such that when the terminal voltage reaches the upper or lower limit of the specified voltage range, the pump flow-rate computation unit determines that the flow rate of the pump is increased by a specified amount, and when the terminal voltage does not reach the upper or lower limit of the specified voltage range, the pump flow-rate computation unit calculates the charge/discharge efficiency of the battery cell from the at least two parameters measured by the measurement unit and, based on the charge/discharge efficiency, determines the flow rate of the pump so that the electrolyte drained from the battery cell is neither overcharged nor overdischarged.