An object is to provide a medical instrument, a medical system, and a mode transition method for medical apparatus which are each capable of unerringly driving a wire in association with a slack in a passive unit. The medical instrument 1 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 21 for pulling the sheath 6, a sheath lock unit 20 for locking movement of the sheath 6, and a control unit 40 for switching among a run-up mode in which the passive unit 2 and the wire driving unit 4 are still disconnected, a sheath free mode in which the sheath 5 is not locked and so set free, a sheath slack adjustment mode in which a slack in the sheath 6 is adjusted to lock the sheath 6 in place and a master/slave mode in which the passive unit 2 connected to the wire driving unit 4 is put into a drivable state.
