Provided is an indwelling needle assembly that allows a more stable procedure to be carried out without malfunctioning. An indwelling needle assembly 1 is configured such that a casing 5 is held while an inner needle 3 and outer needle 21 pierce a target site, and then an inner cylinder operation part 66 of an inner cylinder 6 is pressed in the leading end direction with the thumb or index finger of the hand holding the casing 5, thereby further sending only the outer needle 21 into the blood vessel and making it possible to cancel the prevention of oscillation of an inner needle base 4. By merely pulling a switch operation part 74 of a switch 7 in the base end direction, the inner needle base 4 is oscillated, thereby releasing support of the inner needle base 4 against a compression spring S (energizing member), the compression spring S causes the inner needle base 4 to move in the base end direction, and the inner needle 3 is then housed inside the casing 5.本發明之目的是要提供：不會發生誤作動，又可執行更穩定的手操作之留置針組裝體。 本發明的留置針組裝體(1)係製作成：以手握持殼體(5)而將內針(3)及外針(21)穿刺到對象部位之後，以握持著殼體(5)的手之拇指或食指將內筒(6)的內筒操作部(66)朝前端方向推壓，而只將外針(21)更進一步插入血管內，並且可將原本阻止內針基座(4)進行擺動的狀態予以解除，只要再將開關(7)的開關操作部(74)朝基端方向拉動，即可使內針基座(4)進行擺動，因而可解除受到壓縮彈簧(S)(彈推構件)彈推的內針基座(4)之被支承狀態，並且利用壓縮彈簧(S)將內針基座(4)朝基端方向移動，使得內針(3)被收容到殼體(5)內。
