A secondary battery state of charge determination apparatus comprising: a voltmeter configured to measure an open circuit voltage of a secondary battery; and a current accumulator configured to accumulate charge currents for charging the secondary battery and discharge currents flowing from the secondary battery during a discharge of the secondary battery, respectively; characterized in that the apparatus further comprises: a memory unit configured to store data on relationships between secondary battery capacity decreases since a reference time point and open-circuit-voltage-to-state-of-charge (OCV-SOC) characteristics, and a value related to a secondary battery capacity at the reference time point or a value corresponding to the battery capacity at the reference time point; a control means for controlling the current accumulator to accumulate at least either of the charge currents and the discharge currents in a process in which a secondary battery state of charge changes as the open circuit voltage varies from one level to another level in a region of an OCV-SOC characteristic, in which the OCV-SOC characteristic is less likely to vary due to degradation of the secondary battery since the reference time point compared to another region in which the OCV-SOC characteristic varies due to degradation of the secondary battery; an estimating means for estimating a secondary battery capacity decrease based on the accumulated currents and the value related to the secondary battery capacity at the reference time point or the value corresponding to the battery capacity at the reference time point; a specifying means for specifying an OCV-SOC characteristic based on the estimated secondary battery capacity decrease and the stored data on relationships; and a determining means for determining a secondary battery state of charge based on the open circuit voltage measured by the voltmeter and the specified OCV-SOC characteristic. a voltmeter configured to measure an open circuit voltage of a secondary battery; and a current accumulator configured to accumulate charge currents for charging the secondary battery and discharge currents flowing from the secondary battery during a discharge of the secondary battery, respectively; characterized in that the apparatus further comprises: a memory unit configured to store data on relationships between secondary battery capacity decreases since a reference time point and open-circuit-voltage-to-state-of-charge (OCV-SOC) characteristics, and a value related to a secondary battery capacity at the reference time point or a value corresponding to the battery capacity at the reference time point; a control means for controlling the current accumulator to accumulate at least either of the charge currents and the discharge currents in a process in which a secondary battery state of charge changes as the open circuit voltage varies from one level to another level in a region of an OCV-SOC characteristic, in which the OCV-SOC characteristic is less likely to vary due to degradation of the secondary battery since the reference time point compared to another region in which the OCV-SOC characteristic varies due to degradation of the secondary battery; an estimating means for estimating a secondary battery capacity decrease based on the accumulated currents and the value related to the secondary battery capacity at the reference time point or the value corresponding to the battery capacity at the reference time point; a specifying means for specifying an OCV-SOC characteristic based on the estimated secondary battery capacity decrease and the stored data on relationships; and a determining means for determining a secondary battery state of charge based on the open circuit voltage measured by the voltmeter and the specified OCV-SOC characteristic. a memory unit configured to store data on relationships between secondary battery capacity decreases since a reference time point and open-circuit-voltage-to-state-of-charge (OCV-SOC) characteristics, and a value related to a secondary battery capacity at the reference time point or a value corresponding to the battery capacity at the reference time point; a control means for controlling the current accumulator to accumulate at least either of the charge currents and the discharge currents in a process in which a secondary battery state of charge changes as the open circuit voltage varies from one level to another level in a region of an OCV-SOC characteristic, in which the OCV-SOC characteristic is less likely to vary due to degradation of the secondary battery since the reference time point compared to another region in which the OCV-SOC characteristic varies due to degradation of the secondary battery; an estimating means for estimating a secondary battery capacity decrease based on the accumulated currents and the value related to the secondary battery capacity at the reference time point or the value corresponding to the battery capacity at the reference time point; a specifying means for specifying an OCV-SOC characteristic based on the estimated secondary battery capacity decrease and the stored data on relationships; and a determining means for determining a secondary battery state of charge based on the open circuit voltage measured by the voltmeter and the specified OCV-SOC characteristic.