An air removal mechanism (16) includes a body (38) having a hollow portion (39) capable of storing a liquid (L). In a state where the body (38) is mounted on an introducer sheath (12), a medical device (54) to be inserted into the introducer sheath (12) is insertable via the hollow portion (39). The body (38) has an injection path (44) which communicates with the hollow portion (39) and on which an outlet (44b) is open toward a valve body (32).
