A storage battery system connected to a power system and operative based on a charge/discharge request from an EMS includes: a battery management unit configured to monitor a state of a storage battery including plural cells; a power conditioning system; a controller configured to receive the charge/discharge request and storage battery information from the battery management unit and control the power conditioning system based thereon; a sleep controller configured to receive the storage battery information and execute sleep control to output a charge/discharge command to the power conditioning system so that an average voltage of plural cells is maintained within a voltage range that suppresses deterioration of the storage battery. The sleep controller includes a sleep control stop function stopping the sleep control when a voltage of any of the plural cells in the storage battery information is out of an allowable voltage range including the holding voltage range.