A positioning section that positions a cutting edge, used in cutting, at an indexed point is provided at a portion of a round insert towards a bottom surface at a periphery of the round insert. A support seat, provided at a tool body, includes a first seat side surface, a second seat side surface, and a restraining section. The first seat side surface and the second seat side surface contact and support side surfaces of the round insert that are disposed 90 degrees apart from each other. The positioning section is fitted to the restraining section by bumpy portion fitting. Only one restraining section is provided. A second relief portion that is larger than a first relief portion is provided at a corner at a side of the second seat side surface.