An auto-injection device has a housing with a medicament container disposed within； a dose setting mechanism for setting a dose of a medicament； a drive nut and a plunger rod operably connected, with an energy accumulator operably connected to the dose setting mechanism； and an activating mechanism and a positioning mechanism operably connected to the plunger rod for locking the plunger rod from being displaced. The positioning mechanism includes a locking nut arranged to cooperate with fixed, flexible locking features for locking the locking nut from rotation. A resilient member arranged inside the plunger rod is configured to allow the plunger rod to be resiliently and axially displaced when the locking nut is unlocked.
