PROBLEM TO BE SOLVED: To provide a novel mechanism in piercing a detention needle to an artery and then retreating of an inner needle.SOLUTION: A detention needle assembly comprises: an outer needle holding body 10 for holding an outer needle 18 in an immobile state； and an inner needle holding body 20 for holding an inner needle 28 so as to relatively slide to the outer needle 18. The inner needle holding body 20 is stopped to a protrusion position against to spring energization force, since a push-in button 25 is engaged to inside of a first opening, when the inner needle holding body is at the protrusion position. When engagement of the first opening 11 and the push-in button 25 is released, the inner needle holding body 20 moves toward a retreat position by spring energization force, and is stopped at the retreat position when the push-in button 25 is engaged to a second opening 12. The push-in button 25 may comprise a tip end part which may enter the first and second openings, and a foundation part whose diameter is larger than that of the tip end part, and which does not enter the first and second openings. Therefore, the push-button 25 can be prevented from protruding excessively, from an opening.SELECTED DRAWING: Figure 1【課題】留置針を動脈等に突き刺した後に内針を退避させることに関して、従来にない新たな機構を提供する。【解決手段】外針１８を不動に保持する外針保持体１０と、内針２８を外針１８に対して相対スライド可能に保持する内針保持体２０とを備えた留置針組立体。内針保持体２０は、「突出位置」にあるとき押込みボタン２５が第１開口内に係合することで、スプリング付勢力に抗して「突出位置」に止まる。一方、第１開口１１と押込みボタン２５の係合を解くと、内針保持体２０は、スプリング付勢力によって「退避位置」に向かって移動するとともに、押込みボタン２５が第２開口１２に係合することで「退避位置」に止まる。押込みボタン２５は、第１、２開口内に入り得る先端部と、当該先端部よりも大径で第１、２開口内に入り得ない土台部とを備えることが好ましい。これにより、押込みボタン２５が開口から過剰に飛び出ることを防止できる。【選択図】図１
