The present invention relates to a torsion spring driven injection device for delivering multiple set doses of a liquid drug from a cartridge by moving a plunger forward inside the cartridge. The torsion spring (134) applies a rotational torque onto a drive member (120) engaging a piston rod (115) which is then driven forward by this rotation. The piston rod driver can be coupled and de-coupled from the torsion spring mechanism. In the de-coupled position, the piston rod driver is freely rotatable and can thus rotate if the piston rod is moved axially. A further torsion arrangement (124) is preferably provided such that a torque is build up between the piston rod driver and the housing whenever the piston rod is moved backwards. This torsion arrangement urges the piston rod driver to rotate in a direction bringing the piston rod forward into abutment with the plunger.