The present invention relates to a medicament delivery device comprising a housing (10)； a medicament container holder (60) arranged to accommodate a medicament container (62) provided with a medicament delivery member (64)； a power unit (81) arranged to act on said medicament container (62) for expelling a dose of medicament when activated； an activation mechanism (30, 48, 148) operably connected to said power unit (81), capable of activating said power unit； said activation mechanism (30, 48, 148) comprising a medicament delivery member guard (30) movable in a longitudinal direction in relation to said housing (10)； said activation mechanism comprising a force element (148) arranged to bias said medicament delivery member guard (30) to a proximal extended position wherein said medicament delivery member (64) is covered； locking mechanism (40, 160) operably arranged to lock said medicament delivery member guard (30) in the extended position； that said locking mechanism (40, 160) comprises a number of locking elements (40) connected to said medicament delivery member guard (30) and flexible in a generally radial direction, that said locking mechanism further comprises a number of ledge surfaces (160) connected to said housing, wherein said locking elements (40) flex radially outwards and abut said ledge surfaces (160) when said medicament delivery member guard (30) is moved in the extended position, preventing any further movement of said medicament delivery member guard (30) in the distal direction. The invention is characterised by spaces (158) connected to said housing (10) in which said locking elements (40) are positioned in a non-tensioned state in a delivery mode of said medicament delivery device.
