A battery capacity controller includes a battery capacity estimating part that estimates the battery capacity of a battery in which an internal resistance is increased in accordance with the decrease of the battery capacity in the vicinity of a prescribed lower limit battery capacity and a battery capacity correcting part that changes, during the discharge of the battery, an estimated battery capacity estimated by the battery capacity estimating part to a proper value in a lower limit side when an internal resistance difference as a difference between the internal resistance value of the battery and the internal resistance value of the battery at the time of the lower limit battery capacity is a prescribed value or lower.