An electrostatic discharge protection circuit has a substrate; a first P-well installed on the substrate and having a first P+-doped region and a first N+-doped region, both of which are connected to ground; a second P-well installed on the substrate and having a second P+-doped region and a second N+-doped region, both of which are connected to a power supply voltage; and a third P-well installed on the substrate and having a third N+-doped region, a third P+-doped region, and a fourth N+-doped region, all of which are for input/output signals.