To provide an impact wrench capable of carrying out detection of tightening torque in a high precision and which can be miniaturized and lightened.

This is an impact wrench to add torque to an output shaft 4 by a hammer 31 driven by a motor 1 and rotationally driven. It has a planetary gear decelerator between the motor 1 and the output shaft 4, an impact sensor consisting of a piezoelectric element is provided on a fixed part of an internal gear 11 of the planetary gear decelerator receiving impact reaction force, and tightening torque is detected by integrating output corresponding to impact force detected by this impact sensor.