A cutting insert (10) is configured to include: a first portion (11) having a rake face (14), a flank face (15), a cutting edge (16), and a face (17) facing an insert attachment portion (3) at an end of a tool main body (1); and a second portion (12) having a side surface (12a) including at least one flat plane that is parallel to a center line (C) of a tool or inclined at a prescribed angle to the center line (C). The second portion (12) extends continuously to a region surrounded by the face (17) of the first portion. The second portion (12) is fitted into the tool main body (1) to position the cutting insert (10) and to prevent the cutting insert (10) from rotating.