The present invention relates to an outer cap of a liquid medicine-injecting device, and more particularly, to an apparatus in which an outer cap configured to temporarily form a suction flow passage only upon suction of a liquid medicine and to be removed for opening an injection flow passage upon injection of the liquid medicine is improved in a liquid medicine-injecting device including a filter for filtering foreign substances contained in a liquid medicine, so that firm sealing is established between an inner peripheral surface of the outer cap and a peripheral surface of the liquid medicine-injecting device so as to reliably generate a negative pressure in the injection flow passage and particularly to prevent the liquid medicine from penetrating an injection needle upon suction of the liquid medicine. An outer cap 100 of a liquid medicine-injecting device 200 according to the present invention includes a suction needle 110 for sucking a liquid medicine and a cap hub 120 hermetically coupled to the liquid medicine-injecting device 200, and the outer cap 100 has an internal space extending from the suction needle 110 to the cap hub 120, wherein the internal space is formed with an injection needle-receiving portion 130 for receiving an injection needle 210 when the liquid medicine-injecting device 200 is assembled； and wherein a sealing means configured to be in contact with a peripheral surface of the liquid medicine-injecting device 200 so as to maintain airtightness between the inner space and the injection needle-receiving portion 130 when the liquid medicine-injecting device 200 is assembled is provided at an inlet of the injection needle-receiving portion 130. The outer cap of the liquid medicine-injecting device allows the liquid medicine-injecting device having the filter to more firmly suck the liquid medicine, and particularly fundamentally prevents the injection needle from being contaminated by a liquid medicine that has not yet undergone filtration
