A charge/discharge control device according to an aspect of the present disclosure is equipped with a system frequency measurer 303 that measures a system frequency, a base frequency updater 304 that updates a base frequency, a frequency bias calculator 305 that calculates a frequency bias indicating the difference between the base frequency and the system frequency, a charge/discharge command value decider 306 that uses the frequency bias to decide a power command value, and a charge/discharge controller 307 that causes a power storage system to charge/discharge power. The base frequency updater 304 determines whether or not reverse operation will occur, and in the case of determining that reverse operation will not occur, updates the base frequency so that the base frequency matches a frequency obtained by applying a low-pass filter to a temporal variation of the system frequency, while in the case of determining that reverse operation will occur, updates the base frequency so that the base frequency matches the system frequency.