PURPOSE: To fasten a screw with the actual fastening force remarkably close to the target value by computing the fastening force on the basis of a peak value of the torque pulse and the dependence of an increase coefficient of the fastening force with respect to this peak value of the fastening force.

CONSTITUTION: An impact type screw fastening device main body 1 is provided with a motor 2, of which driving output has a pulse component, a main spindle 3 to be driven by the motor 2, a switch means 4 for driving and stopping the motor 2, and a torque detecting means 5 for detecting a change of the torque of the main spindle 3. A computing means 7 receives the signal from the torque detecting means 5, and computes the fastening force on the basis of a peak value of the torque pulse, which is obtained on the basis of the signal, and the dependence of an increase coefficient of the fastening force with respect to the peak value of the fastening force, and a control means 8 controls the drive of the screw fastening device main body 1 on the basis of the computed fastening force.