To easily position the cutting surface of an insert along a tool holder center line by providing a peripheral positioning member for finely adjusting the peripheral position of the insert.

A flat tip screwdriver, for example, is used to turn a peripheral positioning member 6 via a screwdriver groove formed on the flange 6a thereof, and the axial position of the flange 6a is finely adjusted arbitrarily. As a result, the position of an insert 3 is finely adjusted along a peripheral direction, and the cutting surface 3b of the insert 3 is easily positioned along the center line of a tool holder 1. Furthermore, the cutting surfaces 3b to 3b of each of inserts 3 to 3 in this case are all positioned on the center line at a whirling machining process, and a cutting operation is thereby facilitated.