A safety device for a pre-filled syringe, comprising - a hollow support body (1.2) to retain the pre-filled syringe therein, - a piston rod (2.6) for moving a piston (2.5) within an inner cavity (2.3.1) of the pre-filled syringe, - a hollow needle shield (1.1) that slidable relative to the support body (1.2) and - guiding means for guiding the movement of the needle shield (1.1.) relative to the support body (1.2), wherein the guiding means comprise - a deflectable flexible arm (1.1.4) with a guide pin (1.1.3) radial extending therefrom, the flexible arm (1.1.4) extending essentially parallel to a central axis of the safety device and - a guide track (1.2.2) that guides the guide pin (1.1.3) within and along the guide track (1.2.2) so that the guide pin (1.1.3) follows the guide track (1.2.2) when the needle shield (1.1) is slid relative to the support body (1.2), - whereby the flexible arm (1.1.4) is laterally deflected as the guide pin (1.1.3) moves along a section of the guide track (1.1.2) oriented at an acute angle relative to the central axis, - whereby the flexible arm (1.1.4) is connected to or is integrally formed to the needle shield (1.1) or the support body (1.2) and the guide track (1.2.2) is formed into the other of the needle shield (1.1) or the support body (1.2), characterised in that the guiding means further comprise a longitudinal tongue (1.1.2) that is received in a longitudinal groove (1.2.1) when the needle shield (1.1) is moved with respect to the support body (1.2) in a distal direction, whereby a relative rotation of the needle shield (1.1) and the support body (1.2) is prevented, a longitudinal notch (1.2.8) is formed into an outer surface of the support body (1.2), and an outer body (1.3) is slidably arranged relative to the support body (1.2), wherein an inwardly protruding dent is formed into an inner surface of the substantially hollow outer body (1.3) that is received in the longitudinal notch (1.2.8.), and wherein a proximal end of the piston rod (2.6) abuts with or is an integral part of the outer body (1.3). - a hollow support body (1.2) to retain the pre-filled syringe therein, - a piston rod (2.6) for moving a piston (2.5) within an inner cavity (2.3.1) of the pre-filled syringe, - a hollow needle shield (1.1) that slidable relative to the support body (1.2) and - guiding means for guiding the movement of the needle shield (1.1.) relative to the support body (1.2), wherein the guiding means comprise - a deflectable flexible arm (1.1.4) with a guide pin (1.1.3) radial extending therefrom, the flexible arm (1.1.4) extending essentially parallel to a central axis of the safety device and - a guide track (1.2.2) that guides the guide pin (1.1.3) within and along the guide track (1.2.2) so that the guide pin (1.1.3) follows the guide track (1.2.2) when the needle shield (1.1) is slid relative to the support body (1.2), - whereby the flexible arm (1.1.4) is laterally deflected as the guide pin (1.1.3) moves along a section of the guide track (1.1.2) oriented at an acute angle relative to the central axis, - whereby the flexible arm (1.1.4) is connected to or is integrally formed to the needle shield (1.1) or the support body (1.2) and the guide track (1.2.2) is formed into the other of the needle shield (1.1) or the support body (1.2),