Abstract: A battery capacity measuring device in accordance with the present invention has a fully-charged state detector (80e), a detected current integrator (80a), a divider (80b), and a corrector (80c) incorporated in a microcomputer (80). The fully-charged state detector detects that a battery is fully charged. The detected current integrator integrates current values that are detected by a current sensor during a period from the instant the battery is fully charged to the instant it is fully charged next. The divider divides the integrated value of detected current values by the length of the period. The corrector corrects a detected current using the quotient provided by the divider as an offset. Furthermore, a remaining battery capacity calculating system comprises a voltage detecting unit (50), a current detecting unit (40), an index calculating unit, a control unit, and a calculating unit. The voltage detecting unit detects the voltage at the terminals of a battery.