OUTER NEEDLE ASSEMBLY

An outer needle assembly 10 comprises an outer needle 48 wherethrough an inner needle 16 passes, and a passage forming member 50 connected to the outer needle 48 and including a valve body 64 inside such that a first internal space 78 on a distal side of the valve body 64 is longer than a second internal space 79 on a proximal side thereof. A ventilation path 80 for discharging air to an outside communicates with the first internal space 78 near the valve body 64. The first internal space 78 comprises: a puncture part 56 formed in a puncturing portion of the outer needle 48; a tapered part 57 formed on the proximal side of the puncture part 56 with an inner diameter increasing toward the proximal side; and a small-diameter part 89 formed on the proximal side of the tapered part 57 with an inner diameter smaller than or equal to that of a proximal end of the tapered part 57.