The present invention provides an impact wrench and a bolt-tightening method such that a spring force is applied, through the circumference of a spindle coupled with the output shaft of an electric motor, in the forward direction to a hammer which is capable of forward and rearward movement and rotational motion following the spindle. The hammer and an impact shaft are brought in coaxial mesh alignment by leaving a gap between them in the direction of rotation so that when a bolt to be tightened is inserted into a socket fixed to an end of the impact shaft to permit the bolt to be tightened, the mesh contact with the impact shaft is released as a result of the hammer being lifted up in the rearward direction against the reaction force due to the tightening of the bolt. An impact sensor detects release of the hammer from the impact shaft and an angle sensor measures the angle of rotation of the impact shaft. This permits measurement of the torque of the impact shaft by measuring the amount by which the angle of rotation of the impact shaft advances each time the impact force is generated. The amount by which the angle of rotation of the impact shaft advances from the time at which the measured torque has reached the previously set snug torque value can also be measured so that the power supply to the electric motor is disconnected when the amount of advancement of the rotational angle has reached a pre-defined value of the preset angle of rotation to stop the rotation of the impact shaft through a braking circuit.