An endoscopic treatment instrument system comprises an endoscope having a treatment instrument driving mechanism and a treatment instrument head to be attached to the treatment instrument driving mechanism. The treatment instrument head includes a treatment portion for conducting a treatment in response to an external operation, a first sheath tube portion for supporting the treatment portion at a distal end, and a first operating member for driving the treatment portion by advancing or retracting to or from the first sheath tube portion. The treatment instrument driving mechanism includes a second sheath tube portion engageable with the first sheath tube portion and a second operating member movable forward and backward within the second sheath tube portion and engageable with the first operating member. In this endoscopic treatment instrument system, both the treatment instrument head and the second sheath tube portion of the treatment instrument driving mechanism can pass through the same channel and are easily replaceable, even while in use.
