A piston rod device for use in an injection device for an injection cartridge is provided with a single or multiple thread (3) along a predetermined portion of its length, after which the thread (3) changes into a longitudinal groove (5) for each thread, such that a turning movement of the piston rod will turn into an axial movement after the predetermined length. The thread (3) is shaped as one or more helicoidal grooves in the surface of the piston rod (1), the grooves (3) turning into longitudinal grooves (5) after the predetermined length. A nut element (6) cooperates with the thread (3), and its device for cooperating consists of at least one radial inwardly directed projection (10) for each thread, the projection (10) or projections guiding the piston rod (1) by the groove or grooves forming the thread (3).