The constitution of a battery charging circuit uses a power source circuit which performs a charging operation for the battery, and a current-voltage converting circuit inserted in series with the battery which, when assuming the amount of charging current of the battery for the abscissa and assuming the amount of voltage obtained by converting the amount of charging current into a voltage for the ordinate, shows a first current-voltage converting characteristic having a positive inclination in a range of a charging current amount representing a condition of a charging completion and shows a second current-voltage converting characteristic having a smaller positive inclination than that for the first current-voltage converting characteristic in a range of a charging current amount exceeding the above range and converts the above charging current amount into a voltage amount, and the converted voltage amount is further A/D converted into a digital value, and an over current charging condition and a charging completion condition are respectively detected based on the digital value.