A battery pack includes: one or two or more secondary batteries; a charge control switch that turns on/off a charging current to the secondary battery; a discharge control switch that turns on/off a discharging current from the secondary battery; a current-detecting element for detecting the charging current and the discharging current; a voltage measuring part that measures the voltage of the secondary battery; a control unit that controls the charge control switch and the discharge control unit; and a storage unit that stores an initial internal resistance of the secondary battery. The control unit measures a closed circuit voltage and a charging current during charging, and a first closed circuit voltage after a first waiting time and a second closed circuit voltage after a second waiting time. The second waiting time is longer than the first waiting time.