Medicament delivery device comprising: - a body which comprises a housing and a medicament container holder accommodating a multi-chamber medicament container wherein said housing and said medicament container holder are interactively connected to and movable relative each other, the medicament container holder comprising an attachment means for attaching a delivery member, - a mixing guard mechanism interactively connected to the body for driving the medicament container holder within the housing achieving a reconstitution; characterized in that the mixing guard mechanism comprises - a guard member operably connected to the medicament container holder, - a force member arranged to act on said guard member; - a guard member locking means (24, 42) arranged between said guard member and said medicament container holder, wherein said guard member locking means (24, 42) comprises first engagement members operatively acting on second engagement members arranged to said medicament container holder such that the guard member is locked to the medicament container holder against the force of the force member, whereby, when the reconstitution is achieved, the medicament container holder and the guard member are capable of being moved in relation to said housing from a pre-mix position in which said attachment means protrudes through a proximal end of said guard member for allowing the attachment of a delivery member 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 (24, 42) is activated and released by a proximal area of said housing, whereby said guard member is forced in the proximal direction by said force member for covering the delivery member. - a body which comprises a housing and a medicament container holder accommodating a multi-chamber medicament container wherein said housing and said medicament container holder are interactively connected to and movable relative each other, the medicament container holder comprising an attachment means for attaching a delivery member, - a mixing guard mechanism interactively connected to the body for driving the medicament container holder within the housing achieving a reconstitution; characterized in that the mixing guard mechanism comprises - a guard member operably connected to the medicament container holder, - a force member arranged to act on said guard member; - a guard member locking means (24, 42) arranged between said guard member and said medicament container holder, wherein said guard member locking means (24, 42) comprises first engagement members operatively acting on second engagement members arranged to said medicament container holder such that the guard member is locked to the medicament container holder against the force of the force member, whereby, when the reconstitution is achieved, the medicament container holder and the guard member are capable of being moved in relation to said housing from a pre-mix position in which said attachment means protrudes through a proximal end of said guard member for allowing the attachment of a delivery member 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 (24, 42) is activated and released by a proximal area of said housing, whereby said guard member is forced in the proximal direction by said force member for covering the delivery member.