The present invention provides a DC-DC converter which is connected to a high voltage battery and a low voltage battery to convert a high voltage of the high voltage battery into a low voltage to charge the low voltage battery, including: a connecting unit which connects the high voltage battery and the low voltage battery; a converting unit which converts a high voltage supplied from the high voltage battery into a low voltage to charge the low voltage battery; and a protection circuit which is located between the connecting unit and the converting unit and is turned off when at least one of the high voltage battery and the low voltage battery is erroneously connected, to shut off a current path between the high voltage battery and the low voltage battery.