In a power impact tool for fastening a fastening member, a torque for fastening the fastening member can be estimated without using a high-resolution sensor and a high-speed processor. The power impact tool comprises a rotation speed sensor for sensing a rotation speed of a driving shaft of a motor with using a rotation angle of the driving shaft, a rotation angle sensor for sensing a rotation angle of an output shaft to which a bit is fitted in a term between an impact of a hammer to next impact of the hammer, a torque estimator for calculating an impact energy with using an average rotation speed of the driving shaft and for calculating a value of estimated torque for fastening the fastening member which is given as a division of the impact energy by the rotation angle of the output shaft, a torque setter for setting a reference value of torque to be compared, and a controller for stopping the driving of the motor when the value of the estimated torque becomes equal to or larger than a predetermined reference value set by the torque setter.