A treatment instrument includes a probe including 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 treatment instrument includes a hollow sheath which surrounds the probe, the sheath having a cutout made to expose the treatment portion. The treatment instrument includes protrusion which is provided in the sheath to cover the back portion on a side opposite to the cutout and which has dimensions smaller than the dimensions of the probe in a direction that intersects with a central axis of the probe.
