Injection device for manually penetrating a needle arranged to said device into an injection site and automatically injecting a medicament, the device comprising - a distal housing part; - a proximal housing part into which a medicament container is arranged; - a plunger rod arranged to act on a distal stopper which is arranged inside said container; - a push button protruding from said distal housing part and being interactively connected to the plunger rod; - an activation member comprising tubular flexible locking means releasably connected to said plunger rod, wherein said tubular flexible locking means comprises generally radial inwards directed ledges and that said plunger rod comprises a circumferential groove having a shape as for the ledges to fit into said groove; - drive force means pre-tensioned arranged between a proximal end surface of the activation member and an end wall of the plunger rod; - a locking member coaxially arranged around said activation member and being distally slidable in relation to said activation member between a first position wherein said locking member completely surrounds said tubular flexible locking means holding said plunger rod and thereby said drive force means in a pre-tensioned state and a second position wherein said locking member partially surrounds said tubular flexible locking means; - a guide rod coaxially arranged within the drive force means operably connected to the activation member wherein a proximal portion of the guide rod protrudes through the end wall of the plunger rod determining a first predetermined distance between the end wall and the distal stopper, and wherein when said push button is proximally displaced, it forces the plunger rod, the activation member and the guide rod to be proximally displaced, whereby the tubular flexible locking means comes completely out of contact with the locking member and with the plunger rod, primary allowing the plunger rod to be proximally displaced the first predetermined distance by the force of the pre-tensioned drive force means. - a distal housing part; - a proximal housing part into which a medicament container is arranged; - a plunger rod arranged to act on a distal stopper which is arranged inside said container; - a push button protruding from said distal housing part and being interactively connected to the plunger rod; - an activation member comprising tubular flexible locking means releasably connected to said plunger rod, wherein said tubular flexible locking means comprises generally radial inwards directed ledges and that said plunger rod comprises a circumferential groove having a shape as for the ledges to fit into said groove; - drive force means pre-tensioned arranged between a proximal end surface of the activation member and an end wall of the plunger rod; - a locking member coaxially arranged around said activation member and being distally slidable in relation to said activation member between a first position wherein said locking member completely surrounds said tubular flexible locking means holding said plunger rod and thereby said drive force means in a pre-tensioned state and a second position wherein said locking member partially surrounds said tubular flexible locking means; - a guide rod coaxially arranged within the drive force means operably connected to the activation member wherein a proximal portion of the guide rod protrudes through the end wall of the plunger rod determining a first predetermined distance between the end wall and the distal stopper, and