A remaining capacity calculating section is provided that acquires a discharged capacity of a rechargeable battery based on a discharging current and a discharging time of the rechargeable battery, and calculates a relative remaining capacity of the rechargeable battery based on the discharged capacity and the fully-charged capacity of the rechargeable battery. The remaining capacity calculating section employs the rating capacity of the rechargeable battery or a learned fully-charged capacity as the fully-charged capacity when a high capacity mode is selected, and employs a capacity obtained by multiplying the rating capacity or learned fully-charged capacity by a predetermined factor not more than 1 as the fully-charged capacity when a long life mode is selected.