PURPOSE: To measure the remaining amounts of capacities of a secondary battery or a secondary-battery pack.

CONSTITUTION: A secondary battery 11 is contained in a secondary-battery pack 10. A current (i) is determined by a charge current i1 which is supplied to the secondary battery 11 from a battery charger, and a discharging current i2 which is discharged from the secondary battery 11 and supplied to a host apparatus. The current (i) which flows into the secondary battery 11 is converted into a voltage across the terminals of a resistor R. A hexadecimal code A which is used for computing the residual capacity of the secondary battery is formed with a differential amplifier 14 and an A/D converter 15 at a specified sampling period based on the voltage V across the terminal. The computation is performed with a computing circuit 16 by using the hexadecimal code A. The residual capacity data B corresponding to the residual capacity of the secondary battery 11 are outputted into the external host apparatus.