The present invention relates to an injection device (1) comprising:—a body (3) designed to receive a product to be injected, furnished at its distal end with an injection needle (7) and receiving a piston (5) capable of being moved between a storage position and an end-of-injection position under the effect of a distal pressure exerted on a piston rod (8) to the distal end of which it is attached,—means of protecting the injection needle (7) capable of adopting an initial position, during which the injection needle (7) is uncovered, and a final position, in which the said injection needle (7) is covered,—biasing means (26) aiming at moving the said covering element (6) from its initial position to its final position,—immobilization means (16, 31) aiming at keeping the said covering element (6) in its initial position, the said injection device (1) being characterized in that:—the said piston rod (8) comprises at least a first portion (9) and at least one second portion (10) coupled to the said piston (8), the said first and second portions (9, 10) being operatively coupled together and selectively movable together in the distal direction, at least one of said first portion (9) and said second portion (10) being movable in the proximal direction separate from said other one of said first portion (9) and said second portion (10).