The present invention relates to a device for delivering medicament, comprising a generally elongated tubular housing having opposite distal and proximal ends and said tubular housing comprising a distal housing part and a proximal housing part releasebly connected to each other by first engagement means a container inside said proximal housing part and arranged to contain medicament a threaded plunger rod a drive nut threadedly connected to the threaded plunger rod a drive member having a tensioning wheel accessible outside the distal end of the distal housing part and said drive member being connected to the drive nut via a plunger housing arranged between said drive member and said drive nut spring force means having a first end connected to the drive member and a second end connected to a fixed point ON the distal housing part such that said spring force means is tensioned when said tensioning wheel and said drive member are rotated a manually operated activation means releasable interconnected to said plunger housing by fourth engagement means for providing a rotational locking of the plunger housing in any direction when said drive member is rotated and said spring force means is tensioned, and for releasing said rotational locking when said fourth engagement are moved away from each other, such that said drive member and said plunger housing and said drive nut are rotated in the opposite direction forcing the plunger rod to move axially exerting pressure ON the medicament inside the container for expelling a certain predetermined quantity of the medicament through a delivery member wherein the device further comprises a guide nut arranged with guide ledges which cooperate with longitudinal grooves of the plunger for providing a rotational LOCK but allowing a longitudinal movement of the plunger rod in relation to the guide nut and wherein said guide nut is rotationally locked to the distal housing part by fifth engagement means when the two housing parts are
