A semiconductor device capable of preventing an electrostatic surge without increasing a leak current. In the semiconductor device, a protection circuit for protecting an internal circuit is provided between a source line and a ground line. The protection circuit has a protection transistor of which the drain is connected to the source line and the source and gate are connected to the ground line. The protection transistor is configured by integrally forming two types of transistor structural portions. The latter of the transistor structural portions is longer than the former thereof in gate length. In addition, the sum of gate widths of the latter transistor structural portions is larger than the sum of gate widths of the former transistor structural portions.