PURPOSE: To prevent overcharging of a secondary battery by establishing a wiring between the secondary battery and a voltage detecting circuit independent of the wiring between the secondary battery and the power supply circuit.

CONSTITUTION: A wiring between a secondary battery 1 and a voltage detecting circuit 7 is laid independent of the wiring between the secondary battery 1 and a power supply circuit 6. Therefore, the voltage detecting circuit 7 can accurately measure a true voltage generated between the anode and the cathode of the secondary battery 1 and outputs a detected signal 8 indicating such voltage value to a control circuit 9. Thereby, when charging progresses, a detected value from the voltage detecting circuit 7 gradually increases and reaches the predetermined charging end voltage value. In this timing, the control circuit 9 outputs a control signal for starting the charging operation to a power supply circuit 6. Responding to this control signal 10, the power supply circuit 6 stops the charging operation. Thereby, overcharging of the secondary battery 1 can be prevented.