A control circuit corrects a reference gain by multiplying the reference gain by a correction value set in association with state-of-charge information of a storage battery, and uses the corrected reference gain as a first gain for controlling a DC/AC inverter, to adjust the slope of frequency drooping characteristic. The correction value is a value that gradually decreases with increase in state-of-charge information of the storage battery, the minimum value of the correction value is N1 which is a real number greater than 0 and corresponding to a first state of charge for stopping charging of the storage battery, and the maximum value of the correction value is N2 which is a real number greater than N1 and corresponding to a second state of charge for stopping discharging of the storage battery.