A treatment instrument which is used under an environment filled with an electrically conductive liquid includes a probe, a hollow sheath and an insulating member. The probe has a distal portion to chip a treatment target part by ultrasonic vibration and which allows the distal portion to function as one pole in a bipolar electrode. The sheath surrounds the probe. The insulating member covers the sheath except for a partial region on the distal side of the sheath. The partial region on the distal side of the sheath functions as the other pole in the bipolar electrode.
