An injection device for automatic spring driven injection of a liquid drug, comprising: a dose setting mechanism by which doses of an individual size can be set by a user, and a mechanical dose size display for displaying the size of the set dose, which injection device further comprises: a housing defining an interior space and having a longitudinal window, a rotatable dose dial button coupled to the dose setting mechanism, a rotatable scale drum carrying indicia for indicating the size of the set dose and which scale drum is functionally coupled to the dose dial button to rotate when the dose dial button is rotated to set a dose, the rotatable scale drum being rotatable within the interior space defined by the housing during dose setting, a sliding element coupled to the scale drum, and provided with a window, which sliding element is adapted to slide axially in relation to the housing during dose setting, and through which window the indicia carried by the scale drum is visible such that the longitudinal window and the window in combination with the indicia form the dose size display, Characterized in that, the rotatable dose dial button is axially retained in relation to the housing, and that the sliding element move axially within the boundaries of the housing when the scale drum is rotated. a dose setting mechanism by which doses of an individual size can be set by a user, and a mechanical dose size display for displaying the size of the set dose, which injection device further comprises: a housing defining an interior space and having a longitudinal window, a rotatable dose dial button coupled to the dose setting mechanism, a rotatable scale drum carrying indicia for indicating the size of the set dose and which scale drum is functionally coupled to the dose dial button to rotate when the dose dial button is rotated to set a dose, the rotatable scale drum being rotatable within the interior space defined by the housing during dose setting, a sliding element coupled to the scale drum, and provided with a window, which sliding element is adapted to slide axially in relation to the housing during dose setting, and through which window the indicia carried by the scale drum is visible such that the longitudinal window and the window in combination with the indicia form the dose size display, a housing defining an interior space and having a longitudinal window, a rotatable dose dial button coupled to the dose setting mechanism, a rotatable scale drum carrying indicia for indicating the size of the set dose and which scale drum is functionally coupled to the dose dial button to rotate when the dose dial button is rotated to set a dose, the rotatable scale drum being rotatable within the interior space defined by the housing during dose setting, a sliding element coupled to the scale drum, and provided with a window, which sliding element is adapted to slide axially in relation to the housing during dose setting, and through which window the indicia carried by the scale drum is visible such that the longitudinal window and the window in combination with the indicia form the dose size display, Characterized in that, the rotatable dose dial button is axially retained in relation to the housing, and that the sliding element move axially within the boundaries of the housing when the scale drum is rotated.