The disclosure refers to an injection device for automatic spring driven injection of a liquid drug by which doses of an individual size can be set by a user. The injection device comprises a housing defining an interior space and having a longitudinal window, a rotatable dose dial axially retained in relation to the housing, a rotatable scale drum carrying indicia for indicating the size of the set dose, wherein the scale drum is functionally coupled to the dose dial to rotate when the dose dial is rotated to set a dose, a sliding element provided with a sliding window, which sliding element is adapted to slide axially in relation to the housing during dose setting, and through which sliding window the indicia carried by the scale drum is visible. The longitudinal window and the sliding window in combination with the indicia form a dose size display. The scale drum rotates within the interior space defined by the housing during dose setting. The inner surface of the sliding element is provided with an internal feature engaging an external thread provided on the outer surface of the scale drum The sliding element is further axially guided in the housing such that the sliding element moves axially when the scale drum is rotated. A drive spring is attached to the scale drum with one end and to the housing with another end such that relative rotation between the scale drum and the housing charges the drive spring.
