An injection device includes: a body for receiving a product, a needle and a piston attached to a piston rod movable between a storage position and an end-of-injection position. The device further includes a sleeve for protecting the needle which is movable between an initial position where the needle is uncovered and a final position where the needle is covered, as well as biasing means for moving the sleeve from its initial position to its final position and immobilization means for keeping the sleeve in its initial position. The piston rod includes a first portion and a second portion, the second portion being able to switch between a locked position in which the second portion is fixed to the first portion and an unlocked position in which the second portion is movable with respect to the first portion on a predetermined distance along the longitudinal axis of the piston rod.