The present invention relates to an outer cap of a medicinal fluid injection instrument and, specifically, to a device for reliably forming negative pressure at an injection channel when a medicinal fluid is suctioned and particularly, preventing the medicinal fluid from infiltrating an injection needle by reliably sealing the space between the inner circumferential surface of an outer cap and the circumferential surface of a medicinal fluid injection instrument since the outer cap, which temporarily forms a suction channel only during suctioning of the medicinal fluid but is removed so as to open the injection channel when the medicinal fluid is injected, is improved in the medicinal fluid injection instrument comprising a filter for filtering out foreign matter contained in the medicinal fluid. An outer cap (100) has a suction needle (110) suctioning a medicinal fluid and a cap hub (120) maintaining the sealing of the medicinal fluid injection instrument (200) so as to be coupled thereto, and has an inner space formed from the suction needle (110) to the cap hub (120), wherein an injection needle accommodating part (130) for accommodating the injection needle (210) according to the assembly of the medicinal fluid injection instrument (200) is formed in the inner space, and a sealing means for maintaining the sealing of a space between the inner space and the injection needle accommodating part (130) by coming into contact with the circumferential surface of the medicinal fluid injection instrument (200) according to the assembly of the medicinal fluid injection instrument (200) is provided at the entrance of the injection needle accommodating part (130), and thus the medicinal fluid injection instrument comprising a filter more reliably suctions the medicinal fluid and, particularly, the contamination of the injection needle caused by the medicinal fluid before filtration is fundamentally prevented, thereby enabling the marketability of the medicinal fluid injecti
