Provided is an indwelling needle assembly having a novel structure in which the resistance when the inner needle is pulled out is suppressed and the outer needle is pulled out more effectively together with the inner needle being pulled out. In the indwelling needle assembly 10, the safety mechanism portion 22 includes an outer needle hub engaging portion 54 that engages with the outer needle hub 26 and an engaging portion opposing wall portion 55, and these outer needle hub engaging portions. 54 and the engaging portion facing wall portion 55 are restricted from rotating relative to each other, and either the outer needle hub engaging portion 54 or the engaging portion facing wall portion 55 is integrated with the urging portions 58 and 60. On the other hand, when the inner needle 18 is pulled out from the outer needle hub 26, the block portion 82 engaged with the inner needle 18 and the inner needle engaging portion 84 is applied to the urging portions 58 and 60. The urging portions 58 and 60 are displaced in the direction approaching the inner needle 18 by being moved relative to each other, and the needle tip 16 of the inner needle 18 is prevented from protruding from the safety mechanism portion 22. In addition, the engagement between the outer needle hub 26 and the safety mechanism portion 22 can be released.内針の引抜時の抵抗が抑えられて、内針の引抜きと共に外針が引き抜かれることをより効果的に防止する、新規な構造の留置針組立体を提供する。 留置針組立体１０において、安全機構部２２が、外針ハブ２６と係合する外針ハブ係合部５４と係合部対向壁部５５とを備えており、且つこれら外針ハブ係合部５４と係合部対向壁部５５との相対回転が規制されていると共に、外針ハブ係合部５４と係合部対向壁部５５との何れかは付勢部５８，６０と一体的に変位するようにされている一方、外針ハブ２６から内針１８が引き抜かれることにより内針１８と内針係合部８４で係合されたブロック部８２が付勢部５８，６０に対して相対移動して非当接状態とさせられると共に、付勢部５８，６０が内針１８に接近する方向に変位させられて安全機構部２２からの内針１８の針先１６の突出が防止されると共に外針ハブ２６と安全機構部２２との係合が解除され得るようになっている。
