Automatic injection device with improved syringe retention

An automatic injection device includes an end sleeve and a syringe support mounted slidably with respect to the end sleeve. The syringe support is intended to carry an injection syringe provided with an injection needle and with a cap for protecting the injection needle. The automatic injection device includes a membrane formed by at least two locking elements which are mounted so as to slide substantially radially in the end sleeve between a configuration permitting passage of the cap for protecting the injection syringe, in which configuration limit stops of the at least two locking elements, intended to cooperate with a shoulder of the injection syringe, are moved away from each other, and a configuration permitting immobilization of the injection syringe, in which configuration the limit stops of the at least two locking elements are moved towards each other.