A probe unit includes, a probe configured to treat a bone by ultrasonic vibration, a hollow sheath which surrounds the probe and which has a first portion at a small distance from a central axis, and a second portion at a greater distance from the central axis than the first portion, and a knob configured to rotate the sheath relative to the probe between a first position for insertion between the bone and a living tissue facing the bone so that the first portion is located between the bone and the living tissue and a second position for insertion between the bone and the living tissue so that the second portion is located between the bone and the living tissue.