To provide a screw hole capable of providing a large fixing strength with a cross section equal to or smaller than a conventional cross section, preventing a tightened screw from being loosened, and stably fixing an extrusion to an other member for long period.

This screw hole 1a comprises a pair of symmetrical curved pieces 3 formed integrally with an extrusion of aluminum alloy and projected generally in C-shape in cross section from the surface of a flat piece 2. The centers Z and Z' of an peripheral circle R2 forming each curved piece 3 are positioned on a locus (z) rotated from a line (x) obtained by connecting a center Y to the center X of an inner peripheral circle R1 by an angle of 2 to 15° toward the surface of the flat piece 2 where a cross point formed by a line extended parallel with the surface of the flat piece 2 from the center X of the inner peripheral circle R1 forming each curved piece 3 and a circumference concentric to the inner peripheral circle R1 and having a specified thickness (t) is designated as the center Y.