The present invention relates to a medicament delivery device comprising a generally elongated housing comprising a proximal part (10) and a distal part (12); a medicament cartridge (18) comprising a proximal opening closed by a septum (30), an axially movable distal stopper (26), and a first enclosure (20) wherein a liquid and a gaseous fluid are enclosed; a medicament delivery member (36) attachable to said proximal part (10) and having a non-delivery end (34); a medicament cartridge holder (16) wherein the medicament cartridge is coaxially arranged, said cartridge holder being arranged coaxially movable inside said proximal part and comprising holding means (38, 40) capable of holding said medicament cartridge holder (16) in a non-activated position in which said cartridge holder is releasebly engaged to said proximal part and in which a predetermined distance is present between the septum and the non delivery end; a manual pressure mechanism arranged to be moved from a non-pressure position to a pressure position wherein a pressure is build-up inside said cartridge; and an activation mechanism arranged to interact with said holding means only when said manual pressure mechanism is in the pressure position for disengaging said cartridge holder from said non-activated position to an activated position in which said cartridge holder is axially displaced said predetermined distance whereby said septum is penetrated by the non-delivery end and the liquid is expelled due to the pressure built inside the enclosure.