Threaded fastener wrenching structure

In a threaded fastener wrenching structure, an inclination angle ( alpha ) of an arcuate portion (32) of a fitting protrusion (20) is larger than an inclination angle ( beta ) of an arcuate portion (26) of a recess (16) and a wrenching torque is transmitted through an intermediate portion of the fitting protrusion (20) in an axial direction engaged with an opening edge of the recess (16). As a result, a diameter dimension of an engaged part (38) is larger than that in the intermediate of the recess (16) engaged with a tip portion (20a) of the fitting protrusion (20) in a case of alpha < beta , and a smaller force is applied by a constant wrenching torque to the engaged part (38) in inverse proportion to the larger diameter dimension. This restrains a partial damage or a deformation of the fitting protrusion (20) and the recess (16) at the engaged part (38) or allows a small diameter threaded fastener (10) to be efficiently wrenched by higher wrenching torque. A smaller applied force to the engaged part (38) causes stably wrenched state of the small diameter threaded fastener (10) as a result of a small component force in the axial direction to force apart along an inclination of the arcuate portion (32), namely, restraining a bit (18) from coming out of the recess (16).