The invention relates to a pen-shaped torsion spring driven injection device for apportioning set dose of a liquid drug. A piston rod having an external thread is driven forward by a rotatable piston rod guide engaging the piston rod. The piston rod guide is rotated by a torsion spring encompassed between the housing and the piston rod guide. A scale drum having a thread engaging the external thread of the piston rod is provided, and the scale drum is rotated up and down the piston rod by a proximal located rotatable dose setting member cooperating with the scale drum. The scale drum thus move helically when rotated as the piston rod is held inrotatable during dose setting.
