The application disclosed concerns an injection device for apportioning set doses of a drug from a reservoir (40) to a subject. The injection device comprises a rotatable scale drum (50) which is screwed out from the proximal end of the injection device when a does is set. The rotatable scale drum (50) is coupled to a piston rod drive (100, 130) through a push button (30) and a connector pipe (80). When setting a does, the push button (30) engages the scale drum (50) such that both the push button (30) and the scale drum (50) rotate away from the housing (10). During injection, the scale drum (50) is released from the push button (30) and engaged with the connector pipe (80) in order to rotate the piston rod (120).
