The invention relates to a medicament delivery device comprising a locking mechanism (5) for locking a plunger rod (4). The locking mechanism (5) comprises a locking wheel (6) and locking means (7), said locking means (7) being moveable between an active position where the locking means (7) prevent the locking wheel (6) from rotation and an inactive position where the locking means (7) allow the locking wheel (6) to rotate, wherein said plunger rod (4) extends through an opening in the locking wheel (6); and a resilient member (8) arranged at the moveable locking means (7) and being arranged to act on the moveable locking means (7) to displace said movable looking means (7) towards the inactive position where the locking means (7) allow the locking wheel (6) to rotate.