The present invention relates to an outer cap of a medicinal fluid injecting device. In particular the present invention relates to a medicinal fluid injecting device including a filter for filtering foreign materials included in a medicinal fluid. By certainly sealing a gap between the inner circumferential surface of an outer cap and the circumferential surface of the medicinal fluid injecting device by modifying an outer cap which temporarily forms a suctioning flow path only while a medicinal fluid is suctioned, and opens an injecting flow path by being removed when a medicinal fluid is injected, the medicinal fluid injecting device prevents a medicinal fluid from permeating into an injecting needle, and certainly generates negative pressure in the injecting flow path when a medicinal fluid is suctioned. The outer cap (100) comprises a suctioning needle (110) for suctioning medicinal fluid and a cap hub (120) coupled with the medicinal fluid injecting device (200) while maintaining tightness. The outer cap (100) has an internal space extending from the suctioning needle (110) to the cap hub (120). An injecting needle accommodating unit (130) for accommodating the injecting needle (210) is formed in the internal space according to the assembly of the medicinal fluid injecting device (200). According to the assembly of the medicinal fluid injecting device (200), a sealing means is formed in the entrance of the injecting needle accommodating unit (130) to maintain tightness between the internal space and the injection needle accommodating unit (130) by coming in contact with the circumferential surface of the medicinal fluid injecting device (200). The medicinal fluid injecting device having a filter more certainly suctions a medicinal fluid, and specifically, a medicinal fluid before being filtered is fundamentally prevented from contaminating the injecting needle. Accordingly, the marketability of the medicinal fluid injecting device having a filter and the stabi
