Abstract: The invention relates to a process for manufacturing an injection device of the pre-filled type comprising producing, in a deformable plastics material, a tubular syringe body (101) equipped with an external locking element (104) suitable for deforming radially between retracted and dilated positions, producing a tubular sheath (109) equipped with an internal locking element (110, 111) paired with that of the syringe body (101), fitting the sheath (109) round the syringe body (101), by a relative sliding of these latter permitted by the radial retraction of the locking element (104) of the syringe body (101), and introducing into the syringe body (101) a rigid pre-filled container suitable for bringing the locking element (104) of the said syringe body into its dilated position, enabling an irreversible locking of the sheath (109) to be obtained once it has been brought, after injection, into an advanced, post-use protection position.