Treatment instrument and treatment system

A treatment instrument which is used under an environment filled with an electrically conductive liquid, the treatment instrument comprising:

• a probe which 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;
• a hollow sheath which surrounds the probe;
• and
• an insulating member which covers the sheath except for a partial region on the distal side of the sheath,
• wherein the partial region functions as the other pole in the bipolar electrode,
• wherein the distal portion of the probe comprises a treatment portion which contacts the treatment target part, a back portion provided on the side opposite to the treatment portion, and side portions provided at positions between the treatment portion and the back portion,
• wherein the insulating member comprises a first insulating member which covers an inner peripheral surface of the sheath, and a second insulating member which covers an outer peripheral surface of the sheath,
• the sheath comprises
• a protrusion which protrudes to cover the back portion, and
• a cutout which is made to expose the treatment portion and the side portions of the probe,
• wherein in flat section which is cut in an intersecting plane with an extending direction of the probe, an angle formed by an angular portion located at the intersection of a plane defined by the cutout and an outer peripheral surface of the second insulating member is an obtuse angle.