A semiconductor device includes a first circuit block powered by voltages at first and second power supply terminals, a second circuit block powered by voltages at third and fourth power supply terminals, a first ESD (electrostatic discharge) protection circuit including a first field effect transistor having a source, a drain, and a gate, where the gate and one of the source and the drain are connected to the first power supply terminal, the other of the source and the drain is connected to the third power supply terminal, and a first back gate potential adjusting circuit adapted to adjust a potential at a back gate of the first field effect transistor. The first field effect transistor includes a first conductivity type transistor formed in a first well of a second conductivity type serving as the back gate of the first field effect transistor.