To simply and surely enable charging of a secondary battery despite irregularities of a protective voltage and a charging voltage.

When the terminal voltage of a secondary battery 3 is higher than a reference voltage VL, a voltage detection circuit 21 outputs a detection signal to a constant voltage control signal generation circuit 22, which generates a constant voltage control signal. When the input voltage (charging voltage) of a charging circuit 1 is higher than a reference voltage VH, a voltage detecting circuit 23 outputs a detection signal to a switch off signal generating circuit 24, which generates a switch off signal. When an input voltage is lower than the reference voltage VL, a control circuit 25 supplies a charging current outputted from a charging circuit 1 as it is, to the secondary battery 3. When the input voltage is higher than the reference voltage VL, the control circuit 25 charges the secondary battery 3 by a constant voltage mode through a built-in constant voltage circuit. When the input voltage becomes higher than the reference voltage VH, charging operation is stopped. When a charging current is cut off, the charging circuit 1 decreases the charging voltage to be outputted slightly.