When the inner needle is pulled out from the outer needle, the needle tip of the inner needle can be reliably covered and protected by the needle tip protector without requiring troublesome work such as changing the inner needle from the hub. An indwelling needle assembly having a structure is provided. The indwelling needle assembly (10) is provided with a rotation engagement mechanism for releasably engaging the needle tip protector (22) and the outer needle hub (26) by relative rotation, while the needle tip protector (22) is provided with the inner needle. A rotation restricting mechanism for restricting relative rotation of the needle tip protector (22) and the inner needle (18) around the axis in a state of moving to the tip of (18) is provided, and the rotation of the inner needle hub (20) around the axis is provided. Is exerted from the inner needle (18) to the needle tip protector (22) by the rotation restricting mechanism, so that the outer needle hub (26) engaged with the needle tip protector (22) by the rotation engaging mechanism is relatively rotated. It is possible to leave.内針を外針から引き抜いた際に、内針ハブからの持ち替え等の面倒な作業を必要とすることなく、内針の針先が針先プロテクタで確実に覆われて保護され得る、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、針先プロテクタ（２２）と外針ハブ（２６）とを相対回転により離脱可能に係合させる回転係合機構を設ける一方、針先プロテクタ（２２）が内針（１８）の先端に移動した状態で針先プロテクタ（２２）と内針（１８）との軸回りの相対回転を規制する回転規制機構を設けて、内針ハブ（２０）の軸回りの回転が内針（１８）から回転規制機構により針先プロテクタ（２２）に及ぼされることで、針先プロテクタ（２２）に対して回転係合機構で係合された外針ハブ（２６）が相対回転により離脱可能とされている。
