A step-down DC-to-DC converter includes an input terminal, an output terminal, a switching transistor, and a rectifying transistor. The switching circuit is connected between the input terminal and the output terminal and is configured to reduce an input voltage down to a predetermined voltage and to output the predetermined voltage from the output terminal to an external load. The series circuit includes a switching transistor and an inductor. The switching transistor is connected between the input terminal and the inductor, and is configured to perform a switching operation. The inductor is connected between the switching transistor and the output terminal. The rectifying transistor includes a P-type transistor and connected between a connected point between the switching transistor and a common ground. The rectifying transistor has a gate connected to the connected point.