A secondary battery system comprising: a CCV (closed circuit voltage) acquisition unit which acquires each of a charging CCV at a predetermined charging point during a charging period of a battery and a discharging CCV at a predetermined discharging point during a discharging period of the battery; an SOC (state of charge) calculation unit which calculates a charging SOC on the basis of the charging CCV and a discharging SOC on the basis of the discharging CCV; a gap SOC error reduction processing unit which calculates, on the basis of the charging SOC and the discharging SOC, a corrected SOC having a reduced gap SOC error that is included in an SOC of the battery, and which results from a gap between the CCV and an OCV (open circuit voltage); and a cumulative SOC error reduction processing unit which replaces, at a predetermined replacement point, the SOC of the battery with the corrected SOC calculated by the gap SOC error reduction processing unit, updates the SOC of the battery, and reduces a cumulative SOC error included in the SOC of the battery and resulting from a measurement error of a current value, characterised in that the gap SOC error reduction processing unit includes: a first SOC computation unit which acquires m (m is an integer of one or more) number of first SOCs on the basis of the charging SOC and a fluctuation component of the SOC acquired on the basis of a current value integrated from a predetermined charging point; a second SOC computation unit which acquires n (n is an integer of one or more) number of second SOCs on the basis of the discharging SOC and a fluctuation component of the SOC acquired on the basis of a current value integrated from a predetermined discharging point; and an averaged SOC computation unit which computes an averaged SOC as the corrected SOC by performing averaging processing with use of the m number of the first SOCs and the n number of the second SOCs. a CCV (closed circuit voltage) acquisition unit which acquires each of a charging CCV at a predetermined charging point during a charging period of a battery and a discharging CCV at a predetermined discharging point during a discharging period of the battery; an SOC (state of charge) calculation unit which calculates a charging SOC on the basis of the charging CCV and a discharging SOC on the basis of the discharging CCV; a gap SOC error reduction processing unit which calculates, on the basis of the charging SOC and the discharging SOC, a corrected SOC having a reduced gap SOC error that is included in an SOC of the battery, and which results from a gap between the CCV and an OCV (open circuit voltage); and a cumulative SOC error reduction processing unit which replaces, at a predetermined replacement point, the SOC of the battery with the corrected SOC calculated by the gap SOC error reduction processing unit, updates the SOC of the battery, and reduces a cumulative SOC error included in the SOC of the battery and resulting from a measurement error of a current value, characterised in that the gap SOC error reduction processing unit includes: a first SOC computation unit which acquires m (m is an integer of one or more) number of first SOCs on the basis of the charging SOC and a fluctuation component of the SOC acquired on the basis of a current value integrated from a predetermined charging point; a second SOC computation unit which acquires n (n is an integer of one or more) number of second SOCs on the basis of the discharging SOC and a fluctuation component of the SOC acquired on the basis of a current value integrated from a predetermined discharging point; and an averaged SOC computation unit which computes an averaged SOC as the corrected SOC by performing averaging processing with use of the m number of the first SOCs and the n number of the second SOCs. a first SOC computation unit which acquires m (m is an integer of one or more) number of first SOCs on the basis of the charging SOC and a fluctuation component of the SOC acquired on the basis of a current value integrated from a predetermined charging point; a second SOC computation unit which acquires n (n is an integer of one or more) number of second SOCs on the basis of the discharging SOC and a fluctuation component of the SOC acquired on the basis of a current value integrated from a predetermined discharging point; and an averaged SOC computation unit which computes an averaged SOC as the corrected SOC by performing averaging processing with use of the m number of the first SOCs and the n number of the second SOCs.