To eliminate a variation in residual capacity between cells to prevent reduction in chargeable capacity, and to detect residual capacity during charging with high accuracy.

In a CPU 21 (charge control device) of a lithium ion secondary battery 11 composed of plural cells 11a-11n using lithium metal phosphate with an olivine structure for a cathode which are connected in series, when the output voltage (e.g., Va) of any of the plural cells 11a-11n reaches a predetermined full charge voltage FV, equalization control is exerted by a voltage detection discharge control unit 22 in such a way that the output voltage Vn of a cell with lowest voltage (e.g., 11n) among the output voltages Va-Vn of cells 11a-11n is referenced, to which the output voltages Va-Vm of cells 11a-11m with higher voltages than the lowest are adjusted, so that the output voltages Va-Vn of all cells 11a-11n become equal.