A treatment instrument which is used under an environment filled with an electrically conductive liquid, the treatment instrument includes: a probe comprising a treatment portion which treats a treatment target part by ultrasonic vibration, and a back portion provided on a side opposite to the treatment portion, the probe constituting one pole in a bipolar treatment； a hollow sheath which comprises a protrusion to constitute the other pole in the bipolar treatment and which surrounds the probe； and a cutout which is provided in the sheath to expose the treatment portion of the probe and which comprises portions different from each other in inclination to a central axis of the probe.
