A semiconductor device includes: a MOS transistor; a protection diode; and a semiconductor substrate. The MOS transistor and the protection diode are disposed in the semiconductor substrate. The drain of the MOS transistor is connected to the cathode of the protection diode. The source of the MOS transistor is connected to the anode of the protection diode. The MOS transistor has a withstand voltage defined as V.sub.T. The protection diode has a withstand voltage defined as V.sub.D, a parasitic resistance defined as R.sub.D, and a maximum current defined as I.sub.Rmax. They satisfy a relationship of V.sub.T>V.sub.D+I.sub.Rmax.times.R.sub.D. The maximum current of I.sub.Rmax is equal to or larger than 45 Amperes.