A semiconductor integrated circuit device has an SOI substrate comprising an insulating film laminated on a semiconductor support substrate and a semiconductor thin film laminated on the insulating film. A first N-channel MOS transistor, a first P-channel MOS transistor, and a resistor are each disposed on the semiconductor thin film. A second N-channel MOS transistor serving as an electrostatic discharge (ESD) protection element is disposed on a surface of the semiconductor support substrate that is exposed by removing a part of the semiconductor thin film and a part of the insulating film. The second N-channel MOS transistor has a gate electrode, a source region and a drain region surrounding the source region through the gate electrode to maintain a constant distance between the drain region and the source region.