Injection device for manually penetrating a needle arranged to said device into an injection site and automatically injecting a medicament mixture, the device comprising - a distal housing part; - a proximal housing part into which a multi-chamber container is arranged and wherein said proximal housing part is arranged to be manually displaced into said distal housing part for mixing at least two substances arranged inside said container; - 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; - drive force means pre-tensioned arranged between a distal 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 by said proximal housing part 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 flexible locking means; characterised in that said device further comprises a guide rod coaxially arranged within the drive force means between the proximal end surface of the activation member and the distal stopper, and wherein a proximal portion of said guide rod protrudes through the end wall of the plunger rod determining a first predetermined distance between the end wall and the distal stopper, such that when said push button is proximally displaced, it forces the plunger rod, the activation member and thereby 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 such that the end wall of the plunger rod hits the distal stopper for providing an audible and tactile information that the automatic injection has started. - a distal housing part; - a proximal housing part into which a multi-chamber container is arranged and wherein said proximal housing part is arranged to be manually displaced into said distal housing part for mixing at least two substances arranged inside said container; - 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; - drive force means pre-tensioned arranged between a distal 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 by said proximal housing part 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 flexible locking means;