An operating method of a redox flow battery system that can provide a stabilized output to a system from a redox flow battery system annexed to a wind power generator, to produce an improved battery efficiency. The redox flow battery system comprises the wind power generator 10, a redox flow battery 30 annexed to the wind power generator, and an AC/DC converter 40 connected to the redox flow battery. In the operating method, an output of the wind power generator within a certain period of time in the past is subjected to a first averaging process; a present output value of the wind power generator is subtracted from the resulting value of the first averaging process, from which an input/output command value to the AC/DC converter 40 is determined; the input/output command value is subjected to a variation computing process to find variation without any consideration of a sign of the input/output command value; the resulting value obtained by the variation computing process is further subjected to a second averaging process, and an output command value for a pump P to circulate electrolytic solution of the redox flow battery is determined based on the result of the second averaging process.