An automatic injection device including an injection syringe equipped with a syringe body and with a piston rod mounted movably in this syringe body so as to cover an injection stroke, a piston control member capable of changing between a configuration in which it is in engagement with the piston rod and a configuration in which it is disengaged from this piston rod, mechanism for moving the piston control member between its two configurations with respect to the piston rod. The mechanism for moving the control member between its two configurations includes a camway carried by a disengagement member and a cam carried by the control member. The disengagement member is movable with respect to the piston rod and the syringe body after the end of the injection stroke, in such a way as to cause the camway to interact with the cam.
