The pre-filled single-use hypodermic devices must be provided with a needle cap device (1) during storage for protection of the injector needle (4) and the contents of the hypodermic device which is removed prior to use. The two-part needle cap device (1) includes a plastic outer casing (5) of hard elastic transparent material which is provided inside with a lining member (6) sealing the tip of the injector needle and made from a soft elastic material. The injector head (2) and the plastic outer casing (5) have a coupling device for making a mechanical stable form-fitting connection and/or a positive-locking connection with a reliable microbiological seal when the injector head (2) is inserted in the outer casing (5). This coupling device is formed by elements on or features of the injector head (2) and outer casing (5). The needle cap according to the invention permits a drop-free removal since the injector head and the form of the needle cap cooperate with and conform to each other so that the tip of the needle injector is released from the lining member (6) only when a pressure inside the outer casing corresponds to the external air pressure.