A method for operating a redox flow battery system in which the output to a power system from a redox flow battery system annexed to a wind power generator is stabilized to improve the battery efficiency. The system is provided with a wind power generator (10), a redox flow battery (30) annexed to this wind power generator, and an AC/DC converter (40) connected to this redox flow battery. The output of the wind power generator within a past given period is subjected to a first averaging. An input/output command value to the AC/DC converter (40) is determined on the basis of the result of the subtraction of the current output of the wind power generator from the result of the first averaging. With no consideration of the sign of the input/output command value, a displacement operation is carried out to determine a displacement quantity. The result of the displacement operation is subjected to a second averaging. The output command value of a pump (p) for circulating the electrolyte of the redox flow battery is determined according to the result of the second averaging.