An object is to provide a co-injection member which can smoothly collect a liquid of an internal flow route, which can smoothly inject a liquid into the internal flow route, and which can reliably prevent congestion of the liquid in the internal flow route. A co-injection member (1) includes a main body (2) that can be connected to a blood circuit through which blood is circulated, an internal flow route (±) that is formed inside the main body (2), and that can circulate the liquid of the blood circuit, an inlet flow route (A) that is formed inside the main body (2), and that introduces the blood into the internal flow route (±), an outlet flow route (B) that is formed inside the main body (2), and that can discharge the blood of the internal flow route (±), and a rubber member (4) that is attached to an opening (2c) formed in the main body (2), and that separates the inside and the outside of the internal flow route (±) from each other. The co-injection member (1) can collect the blood of the internal flow route (±) and can inject the other liquid into the internal flow route (±) via the rubber member (4). The inlet flow route (A) extends while a diameter thereof gradually decreases toward a position shifted from a center (P) of the internal flow route (±).
