The medical instrument a wire, a flexible sheath through which the wire is inserted, a wire driving unit for driving the wire, a passive unit that is put into operation as the wire is driven, a sheath pulling unit for pulling the sheath, a sheath lock unit for locking movement of the sheath, and a control unit for switching among a run-up mode in which the passive unit and the wire driving unit are still disconnected, a sheath free mode in which the sheath is not locked and so set free, a sheath slack adjustment mode in which a slack in the sheath is adjusted to lock the sheath in place and a master/slave mode in which the passive unit connected to the wire driving unit is put into a drivable state.