A battery is charged by using in series a constant-voltage DC power supply with current limit function and a constant voltage circuit which has a differential comparator circuit and whose current limit value is variable. Following a period of charging with the current of current limit value by the constant-voltage DC power supply, a period is provided in correspondence with the state of charging of the battery, for charging with a current limit value of the constant voltage circuit which is smaller than that current limit value. The detection of the charging state of the battery is effected by a comparison between a current value of a current source of a differential comparator circuit and a differential output of that differential comparator circuit. It enables to reduce electric power consumed by the control transistor of the constant voltage circuit and to reduce the allowable loss required for the control transistor.