The present invention relates to an medicament delivery device (10) having a proximal (11) and distal end (12) and intended for use in combination with a disposable container (20) having a distal and proximal end and comprising at least one chamber in the proximal end for a liquid medicament encapsulated in the container (20) by a movable plunger (26). The medicament delivery device (10) comprises: an elongated housing comprising a proximal housing section (13) and a distal housing section (14), said proximal housing section is configured to accomodate the disposable container (20)； an elongated plunger rod (30) configured to be in contact with said movable plunger (26)； and a drive mechanism arranged in said distal housing section (14) in order to, when the medicament delivery device (10) is activated, apply a force in a proximal direction via said plunger rod (30) acting on said movable plunger (26), wherein said elongated plunger rod (30) comprises an outer periphery in which at least two axially separated recesses and/or cut-out portions (36) are arranged to prevent that leaking fluid is flowing along the plunger rod to the drive mechanism.
