To provide a boring bar capable of effectively preventing cutting vibration generated at the time of cutting without accompanying complication of structure.

An extended shaft part 13 is extended at a distal end part 12b of a shaft part body 12 of the boring bar 10. In the extended shaft part 13, a clearance 19 enabling stay of cutting liquid is provided in a space with an outer peripheral surface of the extended shaft part 13, and a cylindrical balancer 17 is mounted with play in the shaft direction. The cutting liquid stayed in the clearance 19 is agitated by rotation at the time of the cutting, centripetal force is worked on the extended shaft part 13 by a pressure change of the cutting liquid, and vibration is suppressed on the distal end side in which amplitude becomes largest in the shaft part body 12.