In blood extracorporeal circulation (for example, blood extracorporeal circulation for blood purification treatment such as dialysis or the like), conventionally, an indwelling needle is punctured into to indwell in a blood vessel of a patient, and a blood circuit is connected to the indwelling needle. In general, one indwelling needle indwells in each of an artery and a vein, and these indwelling needles are connected to the respective ends of a blood circuit (see Patent Document 1, for example).
However, it is painful for a patient to have two indwelling needles dwelling in his/her blood vessel throughout the entire period of blood extracorporeal circulation. In addition, when two indwelling needles are used, there is a problem that, should a needle for receiving blood (on the vein side) be removed, a large amount of blood will be poured to the outside. In view of the above, in order to avoid use of two indwelling needles, there has been known a single indwelling needle having a structure in which a unit for connection to a blood circuit is branched into two so that one branched connection portion is connected to an inlet end of the blood circuit and the other to a discharge end of the blood circuit (for example, see Patent Document 2).
However, an indwelling needle having a connection portion simply branched into two requires provision of an inlet pump for introducing blood from the indwelling needle and a discharge pump for discharging blood to the indwelling needle. This results in a complicated structure of a, blood circuit. Moreover, as it is necessary to alternately conduct blood introducing operation and blood discharging operation, poor efficiency in blood extracorporeal circulation results. In order to address the above-described inconvenience, it is preferable to form an indwelling needle having a double lumen structure to ensure separate introducing and discharging passages (see Patent Document 3, for example).