Abstract: A threaded fastener capable of being reduced in size and weight. A pair of side wall surfaces (18) of each of six radially outwardly projecting, torque transmitting portions (16) lie on straight lines passing through the center line (O) in a cross section perpendicular to the center line (O) so as to transmit torque through the side wall surface (18), whereby the direction of a surface orthogonal force N applied to a force-applied point P by a wrenching tool coincides with the direction of an effective wrenching force F. This arrangement reduces an operation force required for rotating the wrenching tool and accordingly the load acting on each of the torque transmitting portions, in comparison with a conventional hexagonal head (100). That is, it is possible to reduce the diameter of the head portion (12) of the threaded fastener (10) and the size and weight of the head portion (12), while avoiding deformation of the side wall surfaces (18) through which the torque is transmitted.