A catheter assembly includes a tubular insertion portion a support portion that is connected to a proximal end side of the insertion portion and has therein a flow passage communicating with an inside of the insertion portion a valve body that is disposed in the flow passage and includes: a blocking membrane configured to inhibit a flow of fluid, and an opening/closing part configured such that the blocking membrane is at least partially openable and closeable via the opening/closing part and an insertion member which is arranged proximal of the blocking membrane and is insertable into the opening/closing part by moving in a distal direction. The opening/closing part is formed as a single slit penetrating the blocking membrane, the slit having a distal end opening and a proximal end opening, and a length of the distal end opening being smaller than a length of the proximal end opening.
