To charge a secondary battery without a need of installing a high- accuracy reference power supply at a charger, without becoming an overcharge state and at a sufficiently high voltage.

A one-shot multivibrator 12 receives an overvoltage signal D from a battery pack 19, and it outputs a signal to a voltage source 11 only for a definite time. During a period in which the signal is input to the voltage source 11, the output voltage A of the voltage source 11 continues to drop. Since a comparator 16 has a hysteresis, an overvoltage signal is kept generated, and a voltage B is dropped so as to follow the voltage A. When the voltage B is dropped by the portion of the hysteresis value of the comparator 16, the overvoltage signal is extinguished, and the voltage B is held at a voltage value at this point of time. The voltage A continues to drop further. When the time decided by the one-shot multivibrator 12 elapses in such a way that a signal from the one-shot multivibrator 12 vanishes, the voltage A of the voltage source 11 is changed to rise again. The above operation is repeated.