The medical instrument 1 includes a wire 5, a flexible sheath 6 through which the wire 5 is inserted, a wire driving unit 4 for driving the wire 5, a passive unit 2 that is put into operation as the wire 5 is driven, a sheath pulling unit 14, 17, 21, 51, and a sheath lock unit 12, 13, 20 for locking movement of the sheath 6.