A high-strength screw (2) includes a threaded portion (7) having a thread (8). The screw (2) includes an inner core (16) as seen in cross-section of the screw (2), the core (16) having a first hardness. The screw (2) includes an outer surface layer (17) as seen in cross-section of the screw (2). The screw (2) includes an unhardening layer (18) forming the outer surface layer (17) in the threaded portion (7), the unhardening layer (18) having a second hardness being reduced compared to the first hardness of the core (16).