Injection device for manually penetrating a needle arranged to said device and automatic injecting a medicament mixture, the device being of the type comprising: - a second housing part; - a first housing part into which a multi-chamber container is arranged and wherein said first housing part is arranged to be manually displaced into said second housing for mixing at least two substances arranged inside said container; - a plunger rod arranged to act on a stopper arranged inside said container; - drive force means capable of pushing said plunger rod for acting on said stopper, - an activation member comprising flexible locking means releasibly connected to said plunger rod for holding said plunger rod and thereby said drive force means in a pre-tensioned state, and a push button protruding from said second housing part; - a locking member being arranged around said activation member and being axially slidable in relation to said activation member between a first position wherein said locking member completely surrounds said locking means and a second position wherein said locking member partially surrounds said locking means after said locking member has been axially displaced by said first housing when said first housing has been displaced into said second housing part; and wherein said flexible locking means are arranged to be released from said plunger rod only after said locking member is moved from the first position to the second position and said push button is axially moved into the second housing part, such that the flexible locking means comes completely out of contact with the locking member; characterised in that said activation member further comprises hook means comprising at least one flexible tongue arranged with an outwardly directed hook at the outer end arranged to interact with radially holding means being an annular ring provided with a circumferential ledge with a shape corresponding to the hook and arranged on the inner circumference surface of the second housing part for locking said push button when said push button is depressed before said locking member is moved from the first position to the second position and thereby avoiding premature activation of the device, said hook means further comprising a protrusion, with an inclined surface, a distance along each tongue. - a second housing part; - a first housing part into which a multi-chamber container is arranged and wherein said first housing part is arranged to be manually displaced into said second housing for mixing at least two substances arranged inside said container; - a plunger rod arranged to act on a stopper arranged inside said container; - drive force means capable of pushing said plunger rod for acting on said stopper, - an activation member comprising flexible locking means releasibly connected to said plunger rod for holding said plunger rod and thereby said drive force means in a pre-tensioned state, and a push button protruding from said second housing part; - a locking member being arranged around said activation member and being axially slidable in relation to said activation member between a first position wherein said locking member completely surrounds said locking means and a second position wherein said locking member partially surrounds said locking means after said locking member has been axially displaced by said first housing when said first housing has been displaced into said second housing part; and wherein said flexible locking means are arranged to be released from said plunger rod only after said locking member is moved from the first position to the second position and said push button is axially moved into the second housing part, such that the flexible locking means comes completely out of contact with the locking member; characterised in that said activation member further comprises hook means comprising at least one flexible tongue arranged with an outwardly directed hook at the outer end arranged to interact with radially holding means being an annular ring provided with a circumferential ledge with a shape corresponding to the hook and arranged on the inner circumference surface of the second housing part for locking said push button when said push button is depressed before said locking member is moved from the first position to the second position and thereby avoiding premature activation of the device, said hook means further comprising a protrusion, with an inclined surface, a distance along each tongue.