According to the invention, a safety device for a pre-filled syringe comprises a hollow support body to retain the pre-filled syringe therein, a hollow needle shield that is slidable relative to the support body and guiding means for guiding the movement of the needle shield relative to the support body. The guiding means comprise a guide pin, a guide track and a cut-out adjacent to the guide track. The guide pin protrudes into the guide track and moves within and along the guide track when the needle shield is slid relative to the support body. The cut-out allows a side wall of the guide track to be deflected in a lateral direction perpendicular to a central axis of the safety device.