An arm 21 has a shape for grasping a part in a circumferential direction of a tool holder 7. The arm 21 is connected to the tool holder 7 with a bolt 23 inserted into a fixing hole 22 provided in a base side portion of the arm 21. A tapped hole 25 used for exchangeably attaching a cutting tool 24 is formed in a tip side portion of the arm 21. A groove part 26 extending in parallel to an axis of the tool holder 7 is formed in a position close to the base portion of the arm 21. Owing to the groove part 26, the arm 21 works as an elastic hinge when an external force is applied, so as to increase or decrease a radial dimension of the cutting tool 24 while keeping its parallelism to the axis of the tool.