The present invention is provided with: a seal member (23) and an outer cylinder needle tip part (14) for blocking, in a fluid-tight state, both end parts of an external peripheral space (16) formed between the external peripheral surface of an inner needle (2) and the internal peripheral surface of an outer cylinder needle (3), and for slidably holding the inner needle (2) in close contact an opening part (17) formed on the external peripheral surface of the outer cylinder needle (3) and used for intravascular communication with the external peripheral space (16) an outer cylinder needle connection part (15) for allowing communication between the external peripheral space (16) and one end of the blood circuit and an inner needle connection part (7) for allowing communication between an internal space (2a) of the inner needle (2) and the other end of the blood circuit. The inner needle (2) is movably held between the protruding position where the tip part protrudes from the tip of the outer cylinder needle (3) and the housed position where the tip is housed inside the outer cylinder needle (3). The inner needle (2) is made to puncture the interior of the blood vessel while held in the protruding position, and is moved to the housed position after puncture to allow blood to be extracorporeally circulated via the outer cylinder needle connection part (15) and the inner needle connection part (7).La présente invention concerne une aiguille qui comprend : un élément de joint (23) et une partie de pointe daiguille cylindrique externe (14) pour bloquer, dans un état étanche aux fluides, les deux parties dextrémité dun espace périphérique externe (16) formé entre la surface périphérique externe dune aiguille interne (2) et la surface périphérique interne dune aiguille cylindrique externe (3), et pour maintenir avec faculté de glissement laiguille interne (2) en contact étroit une partie douverture (17) formée sur la surface périphérique externe de laiguille cylindrique ex
