PURPOSE: To provide a rotary tool and a clamp mechanism therefor which can prevent run out of rotation of a tip of the rotary tool due to change of rotary tools, obtain high cutting accuracy, and are suitable for the changing work by an automatic changer.

CONSTITUTION: A honing tool 1 is provided with a bracket 4 which is held by a clamp mechanism 3 provided in a machining center 2 and a tool main body 6 which is supported on the bracket 4 via a bearing 5 in such a way that it can rotate. The tool main body 6 and a spindle 10 of a machine tool 2 are coupled by a coupling. Even if an axis of the honing tool 1 is inclined for an axis of the spindle 10 of the machine tool 2 when the honing tool 1 is changed, it is possible to prevent run out of rotation of a tip of the tool main body 6 when rotating force is given to the honing tool 1, and change of the honing tool 1 by ATC becomes possible.