The present invention relates to a battery pack having a function for calculating a remaining capacity of a battery cell that can be charged. When a calculated capacity integrated value Y is smaller than a maximum capacity integrated value Ym, a microcomputer (5) incorporated in the battery pack determines the capacity integrated value Y as the remaining capacity data of a battery. When the calculated capacity integrated value Y is the maximum capacity integrated value Ym or larger, the microcomputer determines the maximum capacity integrated value Ym as the remaining capacity data of the battery. Further, the microcomputer (5) corrects a reference capacity integrated value Y0 on the basis of the number of times that the capacity integrated value Y reaches the maximum capacity integrated value Ym.