The present invention relates to an injection device (1) comprising: -a body (3) receiving a product, having a needle (7) and a piston (5) attached to a piston rod (8) movable between a storage position and an end-of-injection position, -a sleeve (6) for protecting the needle (7), movable between an initial position, where the needle (7) is uncovered, and a final position, where the needle (7) is covered, -biasing means (26) for moving said sleeve (6) from its initial position to its final position, -immobilization means (16, 31) for keeping said sleeve (6) in its initial position, characterized in that: -said piston rod (8) comprises a first portion (9) and a second portion (10), said second portion (10) being able to switch between a locked position, in which said second portion (10) is fixed to said first portion (9) and an unlocked position, in which said second portion (10) is movable with respect to said first portion on a predetermined distance along the longitudinal axis of said piston rod (8).
