Abstract: A device for fastening the cap upon the rear portion of the body of a fountain pen or another writing article, comprising an annular member disposed inside said body and provided with axial extensions, each of which is provided with a part arranged to protrude, through a corresponding perforation of said body, with respect to the outer surface of the body itself in order to cooperate with the inner surface of said cap, is described. Each of said extensions is deformable by flexion such as to generate, in consequence of said flexion, pressures between said parts and the inner surface of the cap when said cap is put on said body.