A seal member and outer cylinder needle (OCN) tip part for blocking, in a fluid-tight state, both external peripheral space (EPS) end parts formed between the external-peripheral-surface of an inner needle (IN) and internal peripheral surface of an OCN, for slidably holding the IN in close contact; an opening part formed on the OCN external-peripheral-surface and for intravascular communication with the EPS; an OCN connection part for communicating between the EPS and blood circuit end; an IN connection part for communicating between the IN internal space and other blood circuit end. The movable IN positioned between the projecting position where the tip part protrudes from the OCN tip and the housed position housing the tip inside the OCN. The IN punctures the blood vessel interior while holding the projecting position, and moved to the housed position after puncture allowing blood to extracorporeally circulate via the OCN and IN connection part.