PURPOSE: To charge a secondary battery fully in a short time by making a transition to a constant voltage charging operation when the battery voltage exactly reaching a set level is detected thereby prolonging the charging time with high current through the constant charging operation.

CONSTITUTION: The charging circuit comprises a constant voltage power supply 25 for charging a secondary battery 11, and a protective circuit 12 for the battery 11 inserted between the battery 11 and the constant voltage power supply 25. The constant voltage power supply 25 comprises a voltage detection circuit including a reference voltage generator Vref 2 for detecting the voltage of the battery 11 and an error amplifier 23 and connected with the battery 11 not through the protective circuit 12, and a constant voltage control circuit 22 for controlling the charging current of the secondary battery such that the voltage detected through the voltage detection circuit has a constant value.