A device for automatic injection of a product into an injection site, the injection device comprising: a container having an open proximal end and a substantially closed distal end and being intended to receive the product, and provided at its distal end with an injection needle providing an outlet port of the container, a housing intended to receive, at least partially, the container, the container being movable relative to the housing between an initial position, in which the needle is contained within the housing, an insertion position, distally spaced relative to the initial position and in which the needle is exposed over a predetermined length, and a final position in which the needle is contained within the housing, a piston, provided in the container and movable with respect to the container, temporizing means in the form of a plunger rod having a proximal part and a distal part, the plunger rod coupled to the piston so as to cause the piston to move with respect to the container, the movement of the piston causing the product to be expelled from the container through the injection needle, elastic return means coupled to the housing for moving the container from the insertion position to the final position, and automatic injection means, arranged to automatically cause the plunger rod to push distally the piston, characterized in that the device further comprises retaining means, at least partly formed on the housing and arranged to maintain the container in the insertion position, and deactivating means, formed on the proximal part of the plunger rod and arranged to release automatically the retaining means and cause the container to move to the final position, wherein, when the piston comes into contact with the distal end of the container, under the force exerted by the automatic injection means on the proximal part of the plunger rod, the distal part of the plunger rod is thereby stopped and the proximal part of the plunger is allowed to continue its distal movement under the force exerted by the automatic injection means for a certain time until the deactivating means cooperate with the retaining means to cause the container to move from the insertion position to the final position. a container having an open proximal end and a substantially closed distal end and being intended to receive the product, and provided at its distal end with an injection needle providing an outlet port of the container, a housing intended to receive, at least partially, the container, the container being movable relative to the housing between an initial position, in which the needle is contained within the housing, an insertion position, distally spaced relative to the initial position and in which the needle is exposed over a predetermined length, and a final position in which the needle is contained within the housing, a piston, provided in the container and movable with respect to the container, temporizing means in the form of a plunger rod having a proximal part and a distal part, the plunger rod coupled to the piston so as to cause the piston to move with respect to the container, the movement of the piston causing the product to be expelled from the container through the injection needle, elastic return means coupled to the housing for moving the container from the insertion position to the final position, and automatic injection means, arranged to automatically cause the plunger rod to push distally the piston, characterized in that the device further comprises retaining means, at least partly formed on the housing and arranged to maintain the container in the insertion position, and deactivating means, formed on the proximal part of the plunger rod and arranged to release automatically the retaining means and cause the container to move to the final position, wherein, when the piston comes into contact with the distal end of the container, under the force exerted by the automatic injection means on the proximal part of the plunger rod, the distal part of the plunger rod is thereby stopped and the proximal part of the plunger is allowed to continue its distal movement under the force exerted by the automatic injection means for a certain time until the deactivating means cooperate with the retaining means to cause the container to move from the insertion position to the final position.