Provided is a redox flow battery system including: a cell that performs charging and discharging through the supply of an electrolyte solution; a monitor cell to which an electrolyte solution that is the same as the electrolyte solution is supplied; a current measuring unit that measures a current that is input to and output from the cell; a voltage measuring unit that measures an open circuit voltage of the monitor cell; and a computing unit that computes a state of charge of the electrolyte solution, wherein the computing unit includes a first processing unit, a second processing unit, and a third processing unit, the first processing unit computes an integral value obtained by integrating a current value measured by the current measuring unit, for an amount of time corresponding to a predetermined time constant, the second processing unit computes a corrected voltage value based on a measured voltage value measured by the voltage measuring unit and the integral value, and the third processing unit calculates a first state-of-charge value of the electrolyte solution from the corrected voltage value.