A circuit for protecting a semiconductor device from electrostatic discharge by protecting an internal circuit from electrostatic current flowing into an input/output pad includes a first discharge unit that discharges the electrostatic current to a first power supply line or a second power supply line. A second discharge unit protects the internal circuit from electrostaticity flowing from the input/output pad or the second power supply line. A power clamp unit discharges the electrostatic current, which is discharged to the first power supply line or the second power supply line by the first discharge unit, to the opposite power supply line. A trigger unit drives the first discharge unit and the power clamp unit with first and second detection voltages generated in response to a voltage drop of the discharged electrostatic current.