The invention relates to an injection device for injecting a dose of drug. The injection device comprises a housing, a dose setting mechanism being operable to set a desired dose, an injection mechanism comprising a piston rod (7) adapted to cooperate with a piston positioned in a cartridge containing a drug to be delivered in order to cause a set dose to be delivered from the cartridge via the injection device, a dosage tube (6), and retaining means arranged to prevent axial movement of the dosage tube in a distal direction relatively to the housing during dose setting. The dose setting mechanism comprises a rotatable dose knob (24), operation of the dose setting mechanism causing energy to be stored in a spring member. The injection mechanism is driven by releasing energy previously stored in the spring member during dose setting.