A medical instrument includes a first elongated member including: a fluid lumen through which a fluid is flowable, an aperture opening located in a distal end surface of the first elongated member; a second elongated member including: an insertion lumen in which the first elongated member is insertable, and a discharge section through which the fluid can be discharged; and a movement section configured to form a seal between the first elongated member and the second elongated member in a state where the first elongated member is disposed in the insertion lumen of the second elongated member, the movement section being configured to allow the first elongated member and the second elongated member to move relative to each other such that the discharge section of the second elongated member is positionable on a distal side of the aperture of the first elongated member.