The invention relates to a safety device comprising a supporting sleeve (16) for holding the syringe body (10), a protective sleeve (18), and at least one holding tongue (19, 20) which is rigidly connected to the protection sleeve and can be held by the supporting sleeve (16), in the holding position. The piston comprises a cylindrical skirt (13A) that can end up facing a part of the proximal end (16A) of the supporting sleeve, at the end of the injection stroke, in order to co-operate with an actuating part (19B, 20B) of the holding tongue, and to release same in such a way as to enable a relative sliding of the protection sleeve and the supporting sleeve in order to protect the neddle. This proximal end part (16A) has at least one deflection ramp (30, 32) that can co-operate with the skirt (13A), at the end of the piston stroke, in order to deform the skirt in such a way as to bring part of the skirt of the actuating part (19B, 20B) closer to the holding tongue (19, 20).
