A DC-DC converter operates as a step-down chopper using a main switch element, a sub switch element, an inductor and a capacitor. A sub switch control signal generating circuit discharges a capacitor with a voltage that is proportional to a difference between a voltage of a power supply input unit and a voltage of a power supply output unit when a PGATE signal is at an “L” level, and charges the capacitor with a voltage that is proportional to the voltage of the power supply output unit when an NGATE signal is at an “H” level. The sub switch element is forcibly turned off and reverse flow of an inductor current is prevented even at the time of a light load as a result of the voltage of the capacitor being generated as an NCTL signal. Thus, reverse flow of an inductor current is prevented and a high-efficiency DC-DC converter is provided without the use of a high-speed comparator or any other kind of comparator.