The invention relates to an auto-injector (1) for administering a dose of a liquid medicament (M), comprising: - an elongate outer casing (2) arranged to contain a syringe (3) with a hollow needle (4) and a stopper (25) for sealing the syringe (3) and displacing the medicament (M), the outer casing (2) having a distal end (D) and a proximal end (P) with an orifice (6) intended to be applied against an injection site, wherein the syringe (3) is slidably arranged with respect to the outer casing (2), - spring means (11) capable of, upon activation: - pushing the needle (4) past the proximal end (P), - operating the syringe (3) to supply the dose of medicament (M), and - retracting the syringe (3) with the needle (4) - activating means (10) arranged to lock the spring means (11) in a pressurized state prior to manual operation and capable of, upon manual operation, releasing the spring means (11) for injection, wherein the spring means (11) is a torsion spring (11) grounded at one end (11.1) in the outer casing (2) and at the other end in a first gear member (13) rotatable about a longitudinal axis, wherein the first gear member (13), upon rotation, is arranged for translatively moving a second gear member (16) toward the proximal end (P), the second gear member (16) prevented from rotating and coupled to the stopper (25) in order to push it towards the proximal end (P), wherein the first gear member (13) is engaged with the activating means (10) prior to manual operation in a manner to prevent rotation and disengaged from the activating means (10) upon manual operation.Linvention concerne un injecteur automatique (1) pour administrer une dose dun médicament liquide (M), comprenant : - un boîtier externe allongé (2) agencé pour contenir une seringue (3) avec une aiguille creuse (4) et un bouchon (25) pour fermer la seringue (3) et déplacer le médicament (M), le boîtier externe (2) ayant une extrémité distale (D) et une extrémité proximale (P) avec un orifice (6) destin
