A medicament delivery device comprising: - a housing having a longitudinal axis extending between, and along, a proximal end portion and a distal end portion of the housing, - a container holder arranged in the housing at the proximal end portion, the container holder being adapted to accommodate a medicament container adapted for attachment of a medicament delivery member at one end portion of the medicament container and comprising a first piston sealingly and slidably arranged inside an opposite end portion of the medicament container, and a second piston slidably arranged inside an intermediate portion of the medicament container, - an axially movable plunger adapted to be displaced along the longitudinal axis into the container holder, - an energy accumulating member arranged inside the housing, the energy accumulating member being adapted for displacing the plunger along the longitudinal axis towards the proximal end portion, the plunger comprises an outer plunger rod, an inner plunger rod arranged at least partially inside the outer plunger rod, - a biasing arrangement for actuating the inner plunger rod, - a locking arrangement, and - a release arrangement operably connected to the locking arrangement, characterised in that the release arrangement comprises a manually operable button; - that the locking arrangement in a first state is arranged to lock the biasing arrangement in an energy storing position and in a second state is arranged to release, by the manually operable button, the biasing arrangement from the energy storing position to displace the inner plunger rod along the longitudinal axis in a direction towards the proximal end portion into the container holder; - that the energy accumulating member comprises a main torsion spring for transferring stored energy into a rotational movement for the displacing of the plunger along the longitudinal axis in a direction towards the proximal end portion; and - a fixed member arranged inside the housing, the fixed member being provided with a threaded inner surface, wherein the outer plunger rod comprises a threaded outer surface, the threaded inner surface of the fixed member engaging with the threaded outer surface of the outer plunger rod, wherein a first end of the main torsion spring is connected to the housing and a second end of the main torsion spring is connected to the outer plunger rod and is releasably engaged with the housing. - a housing having a longitudinal axis extending between, and along, a proximal end portion and a distal end portion of the housing, - a container holder arranged in the housing at the proximal end portion, the container holder being adapted to accommodate a medicament container adapted for attachment of a medicament delivery member at one end portion of the medicament container and comprising a first piston sealingly and slidably arranged inside an opposite end portion of the medicament container, and a second piston slidably arranged inside an intermediate portion of the medicament container, - an axially movable plunger adapted to be displaced along the longitudinal axis into the container holder, - an energy accumulating member arranged inside the housing, the energy accumulating member being adapted for displacing the plunger along the longitudinal axis towards the proximal end portion, the plunger comprises an outer plunger rod, an inner plunger rod arranged at least partially inside the outer plunger rod, - a biasing arrangement for actuating the inner plunger rod, - a locking arrangement, and - a release arrangement operably connected to the locking arrangement, characterised in that the release arrangement comprises a manually operable button; - that the locking arrangement in a first state is arranged to lock the biasing arrangement in an energy storing position and in a second state is arranged to release, by the manually operable button, the biasing arrangement from the energy storing position to displace the inner plunger rod along the longitudinal axis in a direction towards the proximal end portion into the container holder; - that the energy accumulating member comprises a main torsion spring for transferring stored energy into a rotational movement for the displacing of the plunger along the longitudinal axis in a direction towards the proximal end portion; and - a fixed member arranged inside the housing, the fixed member being provided with a threaded inner surface, wherein the outer plunger rod comprises a threaded outer surface, the threaded inner surface of the fixed member engaging with the threaded outer surface of the outer plunger rod, wherein a first end of the main torsion spring is connected to the housing and a second end of the main torsion spring is connected to the outer plunger rod and is releasably engaged with the housing.