Medicament delivery device comprising a housing (10) and container holder (12) accommodating a multi-chamber medicament container wherein said housing and said medicament container holder are interactively connected to and movable relative each other; a mixing guard mechanism interactively connected to the body for driving the medicament container holder within the housing achieving a reconstitution, wherein the mixing guard mechanism comprises a guard member (34), a force member (52), a guard member locking means (24, 42) whereby, when the reconstitution is achieved, the medicament container holder and the guard member are movable in relation to said housing from a pre-mix position to a mixed position wherein said medicament container holder has been displaced distally in relation to said housing, at which mixed position said guard member locking means is activated, whereby said guard member is forced in the proximal direction by said force member for covering the delivery member (22).