Probe unit, treatment tool and treatment system

This probe unit (31) is provided with a probe (26), a sheath (33) and a knob unit (45). The probe (26) is for cutting a bone (12), and ultrasonic vibration is transmitted to said probe from an ultrasonic vibrator (36). The sheath (33) is a tube with an ovular cross-sectional shape and surrounds the probe (26). Said oval has a major axis (L) and a minor axis (S). The knob (45) is provided at the base of the sheath (33). The sheath (33) is inserted between the first bone (12) and a second bone (13) of a joint. Then, the minor axis (S) is positioned between the bones (12, 13). When the sheath (33) is rotated 90 DEG about the major axis (C) through operation of the knob (45), the major axis (L) is positioned between the bones (12, 13), increasing the space between the bones (12, 13). Thus, it is possible to secure sufficient space for treating the bone (12). Instead of the ovular cross-sectional shape, the sheath (33) may have an expansion member (51) that is an annular balloon, or linear dilator (52).