In a semiconductor device including two circuit blocks, an ESD protection circuit between power supply terminals (or ground terminals) of the two circuit blocks having the same voltage level as each other is constructed by at least one diode-connected field effect transistor whose back gate potential is adjusted by a back gate potential adjusting circuit. As a result, the absolute value of the threshold voltage and the ON resistance of the ESD protection circuit can be changed in accordance with whether the operation mode is an ESD protection operation mode or a usual operation mode.