The present invention relates to a medicament delivery device comprising a housing having opposite distal and proximal ends； a medicament container holder releasably connected to said housing； a medicament container arranged inside said medicament container holder； a threaded plunger rod arranged to pass through a first inner wall of the housing and arranged to act on a stopper in the medicament container； a lead screw member coaxially connected to the threaded plunger rod by co-acting first slidably-and-rotatably-locked means； wherein said device further comprises a nut coaxially connected to the threaded plunger rod by a treaded engagement between them, connected to the lead screw member by co- acting non-slidable-and-rotatable means, and connected to the housing by co- acting second slidably-and-rotatably-locked means； a primary dose member coaxially rotatable on the lead screw member when the device is in a non- activated state and connected to the lead screw member by co-acting third slidably-and-rotatably-locked means when the device is in an activated state； a locking member fixedly connected to the housing and releasably connected to the lead screw member by co-acting locking means； a first spring force means arranged between the first inner wall of the housing and the nut, wherein the first spring force means is in a pre-tensioned state when said locking means are engaged and the device is in the non-activated state； a secondary dose member rotatably connected to said primary dose member via a pinion gear； dose setting means connected to the primary dose member by co-acting fourth slidably-and- rotatably-locked means, such that when the device is to be set from the non- activated state to the activated state, the dose setting means are manually manipulated in a pre-determined direction, whereby the locking means are released and the lead screw member is distally moved a pre-determined distance by the first spring force means independent of the size of a dos
