A catheter assembly (10) is provided with the following: a tubular external needle (12)； an external needle hub (14) that has an internal channel (26) that communicates with the interior of the external needle (12)； a hemostasis valve (46) that has a blocking membrane (58) that interrupts the circulation of fluid and a slit (60) that enables the blocking membrane (58) to be partially opened and closed； and a plug (50), located closer to the base end of the catheter assembly than the blocking membrane (58), that is inserted into the slit (60) by movement in the tip direction. The slit (60) is formed penetrating the blocking membrane (58) in a straight line and is formed by the relationship L
