A charging device can prevent overcharging by coping with a plurality of cell voltages without increasing a circuit area and current consumption. The charging device selects one of at least two judgment voltages in response to a select signal determined depending on a chargeable voltage of a secondary battery. The charging device compares a comparison voltage based on a voltage of a lower stream of a back flow prevention unit with the selected judgment voltage to detect a fully charged state of the secondary battery. The charging device interrupts supply of charging current to the back flow prevention unit upon detecting the fully charged state.