The purpose is to provide a co-injection member that makes it possible to smoothly collect a liquid from an internal flow path and inject a liquid into this internal flow path, and also makes it possible to reliably prevent the accumulation of liquid in the internal flow path. A co-injection member (1) that makes it possible to collect blood from an internal flow path (α) or to inject another liquid into the internal flow path (α) via a rubber member (4), the co-injection member (1) being provided with: a main unit (2) connectable to a blood circuit through which blood flows； an internal flow path (α) through which the liquid of the blood circuit can flow, the internal flow path (α) being formed inside the main unit (2)； an inlet flow path (A) for guiding blood into the internal flow path (α) and an outlet flow path (B) for guiding blood out from the internal flow path (α), the inlet flow path (A) and the outlet flow path (B) being formed inside the main unit (2)； and a rubber member (4) separating the inner and outer parts of the internal flow path (α), the rubber member (4) being attached to an opening (2c) formed in the main unit (2), wherein the inlet flow path (A) is extended while gradually reducing in diameter toward a position offset from the center (P) of the internal flow path (α).La présente invention concerne un élément de co-injection qui permet de recueillir facilement un liquide depuis une voie d'écoulement interne et d'injecter un liquide dans ladite voie d'écoulement interne, et qui permet également de prévenir de manière fiable l'accumulation de liquide dans ladite voie d'écoulement interne. L'invention porte en particulier sur un élément de co-injection (1) qui permet de recueillir du sang à partir d'une voie d'écoulement interne (α) ou d'injecter un autre liquide dans la voie d'écoulement interne (α) via un élément en caoutchouc (4), ledit élément de co-injection (1) étant pourvu des éléments suivants : une unité principale (2) pouvant être relié
