According to the present invention, a semiconductor device is provided wherein a stepdown-type DC-DC converter includes a first off detection circuit, a second off detection circuit, a capacitor, a capacitor, a diode, inverters, an inductor, a first level shift circuit, a second level shift circuit, a third level shift circuit, a 2-input NAND circuit, a 2-input NAND circuit, a high-side N-channel power MOS transistor and a low-side N-channel power MOS transistor. The first off detection circuit and the second off detection circuit reduce fall times of the gates of the N-channel power MOS transistors, thereby reducing dead time.