A residual capacity calculation apparatus for a battery, comprising: current detection means adapted for detecting a charge/discharge current of the battery; voltage detection means adapted for detecting a terminal voltage of the battery; first residual capacity calculation means adapted for calculating a first residual capacity (SOCi) based on an integrated value of the charge/discharge current of the battery; second residual capacity calculation means adapted for calculating a second residual capacity (SOCv) based on an open circuit voltage of the battery; blend rate setting means adapted for determining a blend rate of the second residual capacity; and third residual capacity calculation means adapted for calculating a post-combination residual capacity (SOC_FIN) by subjecting the first residual capacity and the second residual capacity to weighted combination whereby the blend rate of the second residual capacity (SOC V) is the weight of the second residual capacity (SOC V) characterized in that the blend rate setting means is configured to determine the blend rate of the second residual capacity (SOCv) based on a deviation between the first residual capacity (SOCi) and the second residual capacity (SOCv), such that: the blend rate setting means is configured to increase the blend rate of the second residual capacity (SOCv) during discharging when the first residual capacity (SOCi) is larger than the second residual capacity (SOCv) by a first predetermined amount, and to further increase the blend rate of the second residual capacity (SOCv) as the deviation between the first residual capacity (SOCi) and the second residual capacity (SOCv) becomes larger, and the blend rate setting means is configured to increase the blend rate of the second residual capacity (SOC V) during charging when the second residual capacity (SOCv) is larger than the first residual capacity (SOCi) by a second predetermined amount, and to further increase the blend rate of the second residual capacity (SOCv) as the deviation between the second residual capacity (SOCv) and the first residual capacity (SOCi) becomes larger. current detection means adapted for detecting a charge/discharge current of the battery; voltage detection means adapted for detecting a terminal voltage of the battery; first residual capacity calculation means adapted for calculating a first residual capacity (SOCi) based on an integrated value of the charge/discharge current of the battery; second residual capacity calculation means adapted for calculating a second residual capacity (SOCv) based on an open circuit voltage of the battery; blend rate setting means adapted for determining a blend rate of the second residual capacity; and third residual capacity calculation means adapted for calculating a post-combination residual capacity (SOC_FIN) by subjecting the first residual capacity and the second residual capacity to weighted combination whereby the blend rate of the second residual capacity (SOC V) is the weight of the second residual capacity (SOC V) characterized in that the blend rate setting means is configured to determine the blend rate of the second residual capacity (SOCv) based on a deviation between the first residual capacity (SOCi) and the second residual capacity (SOCv), such that: the blend rate setting means is configured to increase the blend rate of the second residual capacity (SOCv) during discharging when the first residual capacity (SOCi) is larger than the second residual capacity (SOCv) by a first predetermined amount, and to further increase the blend rate of the second residual capacity (SOCv) as the deviation between the first residual capacity (SOCi) and the second residual capacity (SOCv) becomes larger, and the blend rate setting means is configured to increase the blend rate of the second residual capacity (SOC V) during charging when the second residual capacity (SOCv) is larger than the first residual capacity (SOCi) by a second predetermined amount, and to further increase the blend rate of the second residual capacity (SOCv) as the deviation between the second residual capacity (SOCv) and the first residual capacity (SOCi) becomes larger. the blend rate setting means is configured to increase the blend rate of the second residual capacity (SOCv) during discharging when the first residual capacity (SOCi) is larger than the second residual capacity (SOCv) by a first predetermined amount, and to further increase the blend rate of the second residual capacity (SOCv) as the deviation between the first residual capacity (SOCi) and the second residual capacity (SOCv) becomes larger, and the blend rate setting means is configured to increase the blend rate of the second residual capacity (SOC V) during charging when the second residual capacity (SOCv) is larger than the first residual capacity (SOCi) by a second predetermined amount, and to further increase the blend rate of the second residual capacity (SOCv) as the deviation between the second residual capacity (SOCv) and the first residual capacity (SOCi) becomes larger.