Provided is an indwelling needle assembly in which blood retained inside an outer needle base can be completely discharged in a short amount of time. This indwelling needle assembly 1 is provided with: an outer needle 2； an outer needle base 3； a hollow cylindrical member 4； a hemostasis valve 44 which is provided with a freely openable/closable slit 45 and is fitted in the hollow cylindrical member 4； and a connection member 5, of which a proximal end section is inserted into the slit 45 of the hemostasis valve 44 when the hollow cylindrical member 4 is pushed in the direction of the distal end. The outer needle base 3 is provided with a small diameter section 34 and a large diameter section 33 extending further toward the distal end than the small diameter section 34. The hollow cylindrical member 4 has a first notch section 41 on the proximal end side, has air ventilation groove parts 43 that are on the outer circumferential surface and connect a distal end section 42 and the first notch section 41, and can advance from a proximal end position to a distal end position. A second notch section 54 of a protrusion section 53 of the connection member 5 is open in a different direction than the first notch section 41.本發明提供一種能夠將滯留於外針基座內之血液短時間內即完全排出之留置針組裝體。此留置針組裝體(1)具備：外針(2)；外針基座(3)；中空圓筒構件(4)；止血閥(44)，其具有開閉自如之切縫(45)且內嵌於中空圓筒構件(4)；及連接構件(5)，在中空圓筒構件(4)朝前端方向被壓動時，其基端部被插入至止血閥(44)之切縫(45)內。外針基座(3)具備：小徑部(34)、較小徑部(34)於前端側延伸之大徑部(33)。中空圓筒構件(4)其基端側具有第1切缺部(41)，其外周面具有將前端部(42)與第1切缺部(41)連接之通氣溝部(43)，可自基端位置朝前端位置前進；連接構件(5)之突起部(53)之第2切缺部(54)開口於與第1切缺部(41)不同之方向。
