A battery state-of-charge estimation apparatus includes: a storage that stores a charge SOC-OCV curve, a discharge SOC-OCV curve, and internal-division-ratio information; and a state-of-charge estimation unit, wherein: when a capacity difference between a battery before and after charging when the battery is charged when an intersection point is on the discharge SOC-OCV curve or when a capacity difference between the battery before and after discharging when the battery is discharged when an intersection point is on the charge SOC-OCV curve is less than a threshold, the state-of-charge estimation unit references the internal-division-ratio information to determine an internal division ratio; and the state-of-charge estimation unit estimates the state of charge of the battery using the determined internal division ratio, the open-circuit voltage of the battery after elapse of a predetermined time period since the end of charging or discharging, the charge SOC-OCV curve, and the discharge SOC-OCV curve.