A catheter assembly (10) includes a tubular outer needle (12), an outer needle hub (14) having therein a flow passage (26) communicating with an inside of the outer needle (12), a hemostasis valve (46) having a blocking membrane (58) stopping flow of fluid and a slit (60) allowing the blocking membrane (58) to partially open or close, and a plug (50) which is arranged closer to the proximal end than the blocking membrane (58) and is inserted in the slit (60) by moving toward the distal end. The slit (60) is formed in a single slit shape penetrating the blocking membrane (58), and is formed to have a relation of Lf < Lr, where Lf is a linear length at a distal end opening (62) from which the plug (50) protrudes and Lr is a linear length at a proximal end opening (64) in which the plug (50) is inserted.
