To provide a method to directly control axial force by computing the axial force generated by impact force by using an impact wrench as a torque method requires estimation of a torque coefficient and has a difficult point that a computed axial force value also becomes an estimated value though a control method such as the torque method, etc. is conventionally used for screw fastening axial force control.

A 45 degree line is set from an original point 0 of an orthogonal coordinate axis used for axial force control of screw fastening, an impact proceeding point Hi generated by No. (i) impact is detected on the 45 degree line, length HSi of a line segment 0Hi is read and an axial force value Fi after generation of the No. (i) impact is computed by using an expression: Fi=HSicos45.